Single messages can be hidden in random conversations
That's all it takes, cryptographic communication is a list of single messages, if you can pass one without being detected you're done.
If network access is only allowed for bootlocked phones, how would you send those messages but by typing them by hand?
First of all that is impossible, TCP/IP is an open protocol, you can build your own small computer and connect it to a network. But let's for a moment assume this was possible, you can encrypt/decrypt messages with an offline machine and send them to the online one by any number of means, e.g. Build a QR code and scan it with your phone.
That's useful for activists but destroys any ability to organise a movement that needs to recruit followers.
A movement that needs to recruit followers won't encrypt their messages since they need people to be able to read them. They could use public key encryption to ensure that people could send messages to them and they can sign messages, but encrypting the messages is pointless. You only encrypt messages when you trust the other party, otherwise anyone could intercept and encryption is pointless.
Congrats, you now blocked people from sharing urls, setting their delivery address on areas where code has numbers, or prevented people from communicating in a different languages or using slangs since it's impossible to keep track of all of that. Also are you blocking images too? Otherwise you can put text in the image in several different ways.
I also mentioned my home server, but in case you didn't knew you can encrypt files on a VPS, sure there are attacks to access the contents when they're being decrypted by the VPS, but there are lots of ways to have the server send the content encrypted and only decrypt it locally.
How would AI know the difference between "hey check this video " and "hey check this video "? Or even "see you at the party tomorrow" and "see you at the party tomorrow" (i.e. the secret meeting will be tomorrow). Cryptography is so much more than just making your messages look like random characters, hell, using AI it's very possible to have it write a text that uses all of the random letters on a gpg encrypted message in a specific way that others can decrypt, but since hose are still random characters it's impossible to know there's a message there.