@LibM It's been a while since I've had an iPhone 3G or 3Gs, but hopefully these instructions will still be applicable. The screen captures below are from an iPhone 4 running iOS 7.2.1, which does require use of a special configuration profile to set the APN (as does any earlier version of iOS). As it turns out, iPhone 4S running iOS 9.3.5 (latest available for the 4S) should actually not require this procedure if it has the latest carrier profile available for that iOS (Public Mobile 24.0).
I would first confirm which version of iOS you're running (Settings --> General --> About). As I mentioned previously, Version 7.1.2 and lower will require a configuration profile. Version 9.3.5 and up should work with Public Mobile's latest carrier profile. Unfortunately I'm not sure about intermediate versions, particularly iOS 8, since I don't have this to check against.
Regardless of which iOS you have, ensure there aren't any configuration profiles currently installed, since they could interfere with proper functioning of cellular data. To check, go into Settings --> General, and scroll down to the bottom of this section. Above "Reset", you'll either see "Profile" or not. If it's there, it's recommended that you tap on it to go into the installed profiles, and Remove each one.
Next, ensure you're connected to a Wi-Fi hotspot and can access the internet. Launch Safari and go to the URL http://unlockit.co.nz, and click on the "Create APN" option (icon looks like a file box, circled in red below) at the bottom, then select "Canada" for Country, and "Public Mobile" for Carrier. Then click on the "Create APN" button:
This will open a new tab/browsing page, similar to the one below (Received date will be the current date). Click on the "Install" button:
You'll be prompted to confirm installation of the profile. Click on "Install Now". You'll then see the caption "Profile Installed", and you can then click on Done.
You shouldn't need to restart your phone for the settings to take effect, but doing so wouldn't hurt. Check to ensure the new profile has been installed, by going into Settings --> General, and scrolling to the bottom and selecting Profile. You should see something resembling the following:
Turn off your WiFi and ensure that you see "3G" at the top of the screen, near "Public Mobile". In Safari or another browser, ensure you can access the internet to confirm that the APN has been properly set.
Hope this helps!
