Issues with cell service can be SO tricky to isolate because there are a lot of factors not limited to your phone. In fact on newer iPhones, where their logic board is actually two boards sandwiched together, the entire bottom board is devoted just to “Radio” functions–various cellular radios, Wifi, Bluetooth and NFC.
If a network settings reset didn’t resolve, I would want to rule out software altogether by doing a full reset, through a computer if possible (backup first if you need to since a reset will wipe all your data). I usually recommend people not restore anything from backups while trying to determine if the issue is resolved, just set up the phone with the minimum amount of stuff.
If that doesn’t resolve, then I would go for an internal inspection. See if anything looks amiss. Cellular antennas are littered all over. I don’t have a schematic to confirm, but I am pretty sure the antenna mentioned that lives on the loud speaker is the WiFi Diversity antenna, so may not have any bearing on cellular function. The charge port assembly though usually has some antenna components and there are also antennas in the upper left corner, over the end of long skinny arm of the logic board. And near the rear camera (though I believe that is usually Bluetooth).
Just giving your internals a once over may help spot issues.