I had this same issue on T-mobile US. From what I could tell, if I hadn’t sent a message for a few hours, the entire ims system would just stop working. I even observed that, if I had someone send me a message that didn’t go through, if I send out a message right after, I would actually receive the message they sent after a few minutes. Usually, messages are just lost to the ether though. I tried different messengers (stock, fossify, quik) and multiple APN configs. I even copied the APN verbatim from my previous phone which is also a mediatek device. I’ve been using google-less androids for the last decade so it’s absolutely not an RCS or imessage issue. I posted about this issue multiple times in the forum (acknowledged by the team in this thread but never got a follow up.)
Flashing the lineage20 GSI seems to have completely fixed the problem so I can only assume It’s an OSK (Mudita’s stock rom) specific issue, either with the packaged ims driver, or an overzealous battery saver that kills ims after a few hours. But the problem is so intermittent so it’s really hard to debug. The fact that the messages aren’t being redelivered indicates that the phone is acknowledging with T-mobile that the message was received, but then just drops it without committing it to the message database.
I think this issue is being brushed off as another rcs, imessage, and apn post. This is not that; there is absolutely a bug with OSK that’s affecting SMS/MMS reception.