01-03-2019 11:52 - edited 01-03-2019 11:56
First thing to do is remove your sim card and clean it gently with a lint free cloth.
Reinsert and restart the phone.
If this doesn't work, you could pop instore and get a new sim.
You could also try your sim in another phone. If it works then it's a problem with your phone.
If you are posting here via a PC then have a look at this guide and see if it helps.
on 01-03-2019 12:50
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone7 (EE)