Whenever you encounter no service or no network signal on your Android smartphone, one of the first things to check is if you have a valid IMEI and Baseband i.e., neither is unknown or blank.
You can check IMEI by dialing *#06# and Baseband under Settings > About > Baseband (this might vary on some devices. Baseband and IMEI are important for cellular communication so either being blank means no reception.
I have IMEI and Baseband yet no signal, what am I missing?
Aside from null IMEI and unknown Baseband, there are some other factors that could cause network problems.
Location / Poor signal
If you’re in an area where your service provider has no coverage then you’ll surely end up with no service. Test the SIM card on other devices or ask others in the same area using the network.
If the device is locked to a different (or foreign) network then you’ll get a no service message. In this case, the device needs to be network unlocked.
Incompatible firmware or modem file
Flashing an incompatible firmware or one meant for a different region is another possible cause of network problems. A corrupted or wiped modem partition could also be the culprit. The modem file in question could be NVRAM, EFS, QCN, NON-HLOS etc. depending on the chipset. In such a case, you would need to flash a firmware or modem suited for your region or variant.
Your device has a set of frequency bands (in 2G, 3G, 4G or 5G) which it supports, so does your service provider (SIM card). You only get a signal when both support at least one frequency band in common. You can read more on that here. If you had set the device to 4G or 5G only (under Settings > Connection / Network > Mobile Network) then consider setting to 2G or 3G. There’s a better chance they’ll support the same 3G or 2G frequencies.
Not setting the correct APN is enough to cause no service with some service providers. A good example is Ntel Nigeria which is a 4G network.
This could be the SIM card being incorrectly inserted, the SIM being inactive, a SIM slot problem or even a problem on the board. If none of the previously listed possibilities is the culprit then its time to consider it as a hardware issue.