I had a problem with nest wifi and when i called Google they suggested i turn off the Bell modem wifi and or change the name to not be the same as my google nest.....i changed the name and turned off the Bell modem wifi and also factory reset all my mesh points, reinstalled them and to my surprise it worked just fine.....I didnt have to mess with any pppoe or dmz settings....it was pretty much plug and play after that call with google.
one thing you might want to eliminate as a potential issue is your Dlink switch. try running your setup without it and see if it stay up or keeps going down like it does with the switch......if the problem continues without your switch then you know thats not the issue...if it doesnt, then you know your switch is the culprit.