That’s what I thought too. The author appears to have tried every obvious debugging step, except for switching away from US Mobile.
I’ve been using US Mobile myself for a little over a year, and I remember a period of about 2-3 months where most carriers had implemented RCS for iOS on their services, but US Mobile had not, so I couldn’t use RCS for a while. I don’t know what they had to implement to get RCS working on iOS, but it’s possible that their implementation does not work with iOS 26.
I’ve been using US Mobile myself for a little over a year, and I remember a period of about 2-3 months where most carriers had implemented RCS for iOS on their services, but US Mobile had not, so I couldn’t use RCS for a while. I don’t know what they had to implement to get RCS working on iOS, but it’s possible that their implementation does not work with iOS 26.