So this phone is compatible with public mobile and @lokiguy probably needs a roam(hard to find now) or freedom sim card to add the APN to his phone?
I must have look at a different website before when I said not compatible, but the official Oukitel website does say it's comptabile with Public Mobile.
Also, compatibility has already been estabilished as the OP has already said that the phone works for calls and text messages.
Yes, this would appear to be a case of the software on the phone locking changes or additions to APN settings. If anyone is wondering, phone manufactuers do have full control of this. Some phones running Android 8.1 do not allow adding of a new APN while others do. I believe that people need to stop blaming Public Mobile for this (not talking about the OP of this thread but in general). It's entirely the fault of the phone manufactuers. Even if Google had a hand in this, it didn't prevent some other device manufactuers from overrriding this setting to allow new APNs to be added.
This thread may have a solution for you if a freedom sim is not accessible to you or does not work.