I upgraded my phone from Moto G 2nd Generation to Moto G 4th Generation and I am having the same problem. I have already spoken to Virgin Mobile and got no where. They suggested typing in 7890 didn't work then the last digits of my number and again didnt work. She spoke to the Technical guys who sent me an email with links to the Virgin website where I found nothing relevant. Finally clicked on the Community site and found this thread. Has anyone found a solution?
There's been a post on another thread suggesting that 1234 might work.
I set up a new Moto G4 last month and had these messages, I manually checked the (APN) settings and found that they were already correct so just deleted the configuration messages. The owner hasn't had any problems so far.