APN problem and mobile data

Returning from a trip abroad, my Canadian Public Mobile service stopped providing mobile data.  The problem was that the APN parameters for Public Mobile had been erased. And the "edit" and "add" options for APNs was now disabled.  Only entries for TELUS and Koodoo appeared, and neither were suitable.
I found this solution,

• https://forum.xda-developers.com/showpost.php?p=75157321&postcount=14

and it worked.

NB: on the phone

• turn "USB tethering" off
• enable developer mode (system/aboutphone/7 taps) and switch on USB debugging
• make sure you use a good USB cable