I have been experiencing a lot of issues with not receiving SMS and MMS lately. I have verified with people that I am missing SMS messages they send me. Sometimes stuff will come in hours or days later. Sometimes I restart my phone and get a whole bunch all in a row that were sent hours ago. I have seen on this forum that some other people have experienced this issue and there doesn't seem to be clear fix or consensus on what the problem is. I have turned WiFi calling off, as well as have turned off VoLTE, both with no success.
I have a few thoughts on what could be causing this. Have you noticed that you're around a specific place when the messages are delayed? Does your phone have stable signal, or have you noticed it fluctuating?