iMessage won't stay on (iPhone)

I'm helping an elderly family member (not local, so I have to troubleshoot everything remotely). She has an iPhone 12 running the latest iOS. iMessage keeps turning off. Here's the sequence of events:

1. I go into Settings > Messages and see iMessage turned off. I try to switch it on, but it tells me there's an activation error.
2. I go into Settings > FaceTime and similarly, can't activate it.
3. After a bunch of trial and error, I decide to sign the entire phone out of iCloud. I sign out, then restart the phone, then sign back in again.
4. After that, I go back into Settings > Messages and now I can turn iMessage on, and in Settings > FaceTime I can turn FaceTime on.
5. But within a day or two, these end up turned off again, and the entire cycle repeats.

Anyone have any ideas on what to do?

I would tell her to make an appointment at an Apple Store, but they will likely do steps 1-4 then tell her "it's fixed!" when the problem is really step 5. She's elderly, and I don't want her having to make multiple trips.