I see that this was dated in March of last year. Here it is almost a year later and I have the identical problem. Surely someone has resolved this problem at some time.
I have installed a PM sim card and I cannot change the apn from telus to PM.
@tavesone : Did you try all the suggestions in this year old thread you found? There's also an unofficial app to edit the APN on 8.1. There's allegedly a text that the mods might send that sets it as well but that seems rather YMMV as far as a mod that will do it.
What exact make/model/submodel and OS version are you using?
Well known issue with some android phones/manufacturers.
Here is another thread:
As mentioned before, you have different options to solve the issue. Freedom mobile SIM trick, APN app and contacting moderator team.
Didn't work first time around, I'll give it another go. I had no problem with the PM sim card on my Samsung Galaxy. Just on my Oukitel c17 pro.