How to fix RCS issues on iPhone 15 with iOS 18.6.2?

My RCS with Android and other users was just fine, mostly, until last week. Now, with one particular user, my messages don't reach them unless I send from my Mac AND then open imessage on my phone. Super weird and frustrating. I have restarted my phone no less than seven times, trying all the configurations of toggle switches in Settings as I could find on similar posts. And I reset my network settings. I am on T Mobile and it says RCS is on in my Settings.

I am losing a lot of communication, both work and personal with this!

[Re-Titled by Moderator]

Original Title: RCS working on iPhone 15 with iOS 18.6.2 just fine last week. This week, it's in the boot.