Communications: All wireless communications are, theoretically, subject to interception (ie, recording), decryption (via backdoors, quantum computing, poor programming), and interference (passive and active jamming, for example concrete structures blocking signal)

Communications that depend on a third-party to operate may be shut down. This would include, for example, cellular phones (voice/data), Internet connections (such as hotel wifi, LTE/5G), satellite phones.

HAM radio does not depend on third parties, but is not encrypted (iirc encrypting it is illegal?) and can be jammed (as it is a wireless frequency). Licenses are required by FCC in non-life-threatening emergency situations. And you are supposed to give your call sign.

I recommend some type of failover system, but not a uniform one. That is, to ensure that it is hardest to intercept or interfere with our communications, we should not all be using the same technology and applications. This gives a wider "attack surface" for malicious opponents (Antifa, Three Letter Agencies). Meaning any flaws in whatever is in use could cripple the entire "operation".

Anonymity, security and reliability are the three most important factors. Not necessarily in that order.

Phones are relatively reliable unless there is a network overload (likely with many people trying to use it) or shutdown (also likely government/corporate-ordered in an attempt to disrupt communications). They are not anonymous but can be pseudonymous. Like if paid with cash, disguised, no fingerprints or accounts linked, never used elsewhere before or after by same owner. (There is probably more to it than this)

Satellite phones are less likely to be shut down or jammed, as the government uses them for their own communications (Which could be on separate frequencies or satellites). But they are less anonymous, harder to find, and probably more "suspicious" since they look nothing like regular phones (much larger antenna, non-smartphone appearance). They are also subject to government and operator interference, and if your contact in DC does not have one, and cell networks are offline or jammed, and landlines aren't working, they are kind of useless. They are great if local (nearby) infrastructure is damaged or broken since the satellites are much harder for a non-state adversary to take out. Satphones are actually illegal in some countries for a similar reason.

HAM radio and other licensed and non-licensed frequencies such as GMRS, MURS, and FRS are good. The last 3 are shorter range, lower Transmit power than HAM radio, but that can be good for communications between nearby people. Some of these do not require a test, and iirc FRS does not require a license at all (depending on Transmit power and Channels used). Licenses are not anonymous.

The plus of HAM radios is they can transmit on a wide variety of frequencies (much more than the others) and they do not depend on someone else's infrastructure (and their possible compliance with a "shut it all down" order).

Two other communications methods are WiFi and Bluetooth. Mesh networking makes it possible for devices running the same software to connect to each other (this is not anonymous due to the use of MAC Addressing [hardware addresses] for devices). This can allow a message to be transmitted between devices over longer distances if intermediate devices are also running the same app. This would require all devices to be within range of each other, running the same app, and with Wifi/bluetooth on. Without intermediate devices to "boost" the signal, the range of bluetooth is generally about 30 feet and Wifi about 100 feet (depending on obstacles and other devices). These setups do not require permanent or difficult to carry infrastructure. Unlike, for example setting up laptops with long-range wifi antennas and repeater stations. For some of these apps search my other replies, I listed several. Firechat and Briar are 2 of them.

WiFi transmissions can be easily hacked, even with WPA2, and it is meaningless to put a password on it. Software like Aircrack can bypass standard security in a few minutes. Moreover, securing the wifi would discourage others from joining it, and a pre-shared password would defeat the entire purpose. Transmissions over wifi, however, could be encrypted.

Another setup is the GoTenna, a purpose-built hardware device that transmits data over radio frequency in a mesh network type setup between mobile devices running their app. It's expensive (~$200/2 units), and you must have a device in order to use it. The devices can be charged by USB, and they are not particularly large. They are not, however, entirely anonymous.

Powering your stuff: If you are using any gadgets, be sure to do the following: Fully charge the devices. If they have an (easily) user-replaceable battery, purchase a spare battery and bring it. (If you are traveling by commercial aviation, bus, or train, this may not be possible due to battery restrictions - this also applies to larger "battery banks" as well as deep-cycle "wet" batteries) If you can't replace the battery, bring an external power source. These are often sold as "battery generators" or "battery banks". Some have built in inverters so you can use your device's original power adapter. They also often have USB. Some can connect to small solar panels to charge. (A small solar panel may be worth it if you are able to bring it). If your device uses standard-size (AA, AAA, C, D) batteries, buy extras, preferably rechargeable. You can get a quick charger (an IC controlled one is best) that will juice them up in under an hour.

Securing your devices: If possible, do not bring your main devices or any device with personal data on it. Do not login to Apple icloud or Google (Gmail/google play), or any other service on it that you do not wish for your other data to be tied to the event. For android, you can download "apk" files for any apps you may need off the Internet rather than via Google Play. Don't even set it up until you get to DC, if possible. Encrypt it with a different passcode or security password than you use for other devices. Ensure that you use the "full disk encryption" on your phone so that if an incorrect password is entered it locks and wipes itself. Do not use "face unlock", "face id", iris scan, retina scan, fingerprint scan, voice unlock, "google smart lock" or anything that operates based on something you HAVE rather than something you KNOW. You can be compelled to do any of these things to unlock it but a password is not considered to be evidence to turn over (unlike fingerprint, face unlock, iris scan). Don't connect your phone (via wifi or cellular) at the place you are staying if possible. Because there is a record of you paying for it most likely and then when the same phone is "seen" again in DC, you can be traced back.

Accessories: Your phone should have a good case on it. Hybrid cases are more durable than the basic silicone cases, and protect it better. Get a "tempered glass" screen protector, these are more durable against shocks and drops. If your phone supports memory cards, get the highest capacity memory card you can. Some support up to 2 TB. Video uses a lot of storage. SanDisk is a good brand if you buy genuine. Be careful what you record.