The OnePlus 6 No Service error that may show up on the display is one of the worst things you can experience because it puts you in the impossibility of making and placing calls. Usually, when this happens, you should consider a Wi-Fi or GPS problem (these tend to shut off your radio signal when they are having troubles).
When you keep seeing the No Service error, consider that…
Your carrier might be having some problems in that area – especially if you’re in a remote area, try to see if other people who use the same carrier have the same problems or if their signal is fine.
Your radio signal might be shut off – test by typing right into the Dial Pad (*#*#4636#*#*) and have a bit of patience for the Service Mode to automatically load on the screen. Next, navigate to Device Information (or Phone information) and initiate a ping test from the option Run Ping Test. Finally, hit the button Turn Radio Off and expect the OnePlus 6 to restart when you confirm with Reboot.
Your phone’s IMEI number could be compromised – if the carrier really did cut off your service signal, it could be because of a nulled IMEI number or even of what it sees as an unknown IMEI number. Get in touch with them for extra support.
Your SIM card might be deteriorated – or simply not perfectly fitting its dedicated slot. Take it out and try using a different SIM card, at least for comparison. If that one works, put your SIM card back in and see if you’ve fixed the No Service error on the OnePlus 6.