First, let me ask this. When your SIM card from Bell was in your phone. Did you receive a text requesting to port your number? If so, did you reply YES? If you did then the port should have gone through. If you didn't say YES, then the port didn't go through. To confirm this, please place your Bell SIM back in your phone. Do you see Bell on the top left corner? If yes, make a call to see if it goes through. If it does, the port was not successful. If you don't see the word Bell in the top left corner, still make a test call. If it doesn't go through then the port has been sent.
Now install your Public Mobile SIM card. Like @JRod said, go into your settings, find network settings and uncheck automatic. And then reboot. This will reset your network and try to find Public Mobile. You should see Public Mobile as an option now and your phone should work.
If it's still not working, then please follow @Handy1 by calling the number he will provide.
