After each troubleshooting step, check to see if your issue is fixed.

If 
If you see 

Turn your phone off and then on again.
Press & hold the Power button, then touch Power off. Then turn it on to establish a new connection with the network.
Do one of the following:
If your phone allows dual SIMs, disable the SIM, then enable it again. Go to Settings > Network & internet > SIMs & mobile network. Turn the SIM off 

If one of the two SIMs is working, remove the SIM that is not recognized and try it in the other SIM slot. If it doesn't work in the other SIM slot, then the issue is with the SIM card and you need a replacement from your carrier.
If you're having network issues:


Turn your phone off and then on again.
Press & hold the Power button, then touch Power off. Then turn it on to establish a new connection with the network.
Outbound calls
Incoming calls
If you have a signal and can make calls, but callers tell you they consistently get a busy signal when calling you, check that you haven’t blocked their numbers.