Petro Canada Mobility Prepaid iPhone subscribers can use optional cellular data, but you must install a proper APN Profile to enable cellular data on iPhones (to access Internet).

The FAQ on Petro Canada Mobility website refers to a website that has not been updated since 2018 and it does not work for iOS 15.

To fix that serious problem of no internet access, I found a website that works.

Open iPhone's Safari web browser

https://ogne.tech/apns/generator/

In the APN field, enter     rogers-core-appl1.apn

[note spelling: appl as in apple without e, followed by a numeral 1]

Tap INSTALL button.

After APN profile is downloaded. Close Safari.

Go to Settings...

You will see an item like "Profile Downloaded". Tap on it and you will see "APN Ogne Tech"

Tap "install" and follow the on screen prompts. A few steps later, the proper APN profile is installed. Watch magic happens.

Petro Canada Mobility prepaid cellular data is very expensive, $0.15 per MB, only enable cellular data when you occasionally need to use data.
Otherwise turn off cellular data: Settings...Cellular...Cellular Data (ON or OFF)

Notes:

1. Simply entering rogers-core-appl1.apn in the APN field will not work.
(Settings...Cellular...Cellular Data Network...APN). 

2. Ogne Tech's APN profile is signed by a COMODO RSA digital certificate that expires on Feb 11, 2029

3. To remove/delete an APN profile, Settings...General...VPN