I purchased an unlocked Google Pixel 3a XL four months ago and put the AT&T SIM from a BYOP kit in it and everything worked great after the initialization. About a month later, I was not getting a verification code via SMS to reset a password and then noticed I hadn't received other texts in days. I sent out test texts and everyone received them. I sent a text to myself and it didn't come back!
I spoke with the Google rep and he had me reset my WiFi, Mobile & Bluetooth via the Reset options menu, then clear the cache from Messaging (default SMS app), Settings app and another one. I then sent a test SMS to myself and it came right back. The next day, it stopped working again. Then, I received a mass of 15 texts from various numbers all at once that were sent in the previous days.
When this started, my phone was on Android Pie and getting all security updates. Now it is on Android Q and still not working. I have tried other SMS apps, changing the APN settings, network resets, removing contacts and threads of contacts and nothing seems to help. Every once in a while, I will get a text, but I can no longer rely on SMS, which I mainly only use for verification purposes and to be notified that orders are ready to be picked up, etc.
All other data works great at LTE speeds even. Phone calls ALL go through H+ for some reason though, but they still go out at least.(Someone told me you have to have an actual AT&T SIM to make LTE phone calls on their network)
But my number one issue is not receiving texts in any usable fashion. Texts that arrive days or a week later are completely useless.
