Settings for iPhone while abroad

My kid is in Peru for four months. We struggled with how to set up the phone for that time, we maybe should have just paid for the Verizon international plan. But we thought we had a "hack." He also needed a local phone number, which was required by the program.

So, he had a local eSim added, and he turned his US line off. We are still paying for US service for his dormant Verizon line. Somehow he managed to stay in the iMessage groups he was in, and participate, this worked for a month. Iam not sure exactly what his settings were, but it worked. I assume it was while he was on wifi, not using the local cell?

Then, last week he got a notice pop up on his phone that he needed to use his US phone line, or be deactivated. Around that time, we all started getting weird messages in the iMessage groups about people adding/deleting his number, and about changing the group name back to its original name. Around that time also, his messages showed up first as text/RCS then as test/SMS.

Now my one-on-one thread with him shows as text/sms, but he is not actually even receiving them.

Ok, all of that is weird enough. Weirder yet, there is one iMessage group that he still works properly in, and two he doesn't work in. And Me and my son's one-on-one message don't work with him, but my wife does. Our setting are identical, my mind is blown.

I am about to embark into the tech support dungeon at Verizon. I suspect there's three parts to the answer, but it will take hours and multiple calls to get there: (1) they need to fix something on the account side, (2) we need to rebuild all the groups and lose the chat history, and (3) it will happen again in a few weeks anyway... wish me luck.