Check Your SIM Card
Your iPhone’s SIM card is probably not something you think about very often. And while problems with a device’s SIM card aren’t too common, there’s always the chance that your specific card is causing cellular reception issues on your device.
One of the most basic things you can try is to simply power off your iPhone and remove its SIM card tray. Then plug it back in. (eSIM users have to settle for a full restart).
In the unlikely scenario that some type of damage or bug is causing your issues, you can also try requesting a new SIM card from your carrier.