Not all phones are compatible with all networks. Assuming your phone recognizes a SIM card is installed, then with Android:
Go to your settings - Network and internet - Mobile Network - Advanced - Network
Once here, unselect "Automatically select network". Your phone will automatically search or you must search to "choose network" and see if Public Mobile's network is available. If "Public Mobile" doesn't appear and your SIM Card is properly installed, you have a compatibility issue. Try your SIM in another phone, friend's phone, etc and see if the issue persists. There are websites where you can check phone compatibility but this does take a bit of expertise unless your phone is very mainstream.
