RCS Messaging stuck on "Waiting for activation..." breaking RCS group chats (iPhone 15 Pro from T-Mobile)

I recently noticed that I have not been receiving RCS messages in my RCS group chats for some unknown reason. The issue apparently began about two weeks ago (~May 23) but I did not notice until four days ago (~June 3). I can send messages using SMS as fallback and I'm told my Android friends are receiving them, but any message sent back by an Android phone in these existing group chats sends back an unreceivable RCS message. Ideally, I want to fix my RCS, but I am willing to accept a stop-gap solution that simply allows full SMS fallback. Turning off RCS in Settings does not allow full SMS fallback like it has in the past, like when RCS first launched for me.

I have spoken with about six T-Mobile service representatives to find a fix to this issue, as I believe it is networking-centered. They claim there are no networking issues on their end based on their engineers' reports, but a self-diagnostic through Apple's Support app suggests that the issue is networking-focused.

I have reset my eSIM and completely reset my iPhone (wiped then restored from iCloud backup), and the RCS setting still says "Waiting for activation..." I have tried everything that T-Mobile suggests and everything that I can find online.

My next step is to go to an Apple Store to see if this could possibly be a software issue, though I doubt that is the case. Someone vaguely mentioned that my downloading Whatsapp around May 23 could be connected, but I doubt that after having reset my whole phone with no fix. I am on iOS 18.5 with the latest carrier settings. T-Mobile suggests just waiting for an iOS update and hoping that helps, but I don't see that as a solution.

Does anyone have any insights or anything else I could try?