As for your question about ports on the VMG1312-B10A, Burakkucat or Johnson or one of umpteen experts may be able to pitch in here and help out, as I am no expert - I was guided by these sages and the config was originally written by AA who pre-configured the B10A for me and I then perverted their config.
Since I have so many modems, I have a program I wrote which adapts a master config template edits it and customises it ready to be pushed into modem n=1,2,3,4 with per ‘slot’ customisations. I sort-of regret doing that, having the config file for all the modems different, but it’s easier for me to use, which is frequent, and more of a pain for setup, which is rare event. If I put the wrong modem in the wrong slot, they all work as far as basic internet access goes since pppoe cares nothing about IP or IP addresses, but I then can’t administer them with my router if they are in the wrong slots because in my case the routing is confused by the modem’s admin addresses being wrong.
(Btw You might find that old thread, and another one that preceded it, interesting reading.)