Wifi dies when certain PCs connect

Hi, so I have a real head scratcher here...

Background:

I was an AT&T customer for the last year, moved in to my own place 2 weeks ago and carried the ATT service over with me, BUT, best they could do was 50mbps, so I cancelled and switched to xFi. Up to this point had no issues with any connectivity.

The story:

On Friday I received my gateway and all set up well, and worked for an hour or 2, then wifi suddenly dissappeared. At this point EVERYTHING was on wifi. Reboot the gateway, wifi returned. Saturday morning found wifi dead again, when in to the chat with a rep, he did some signal resetting, reset the gateway, did some stuff, and it worked, for an hour or so... Later on Saturday I returned the gateway and got the new X7 or whatever and from the start issues with gateway wifi. Again on the chat , spend an hour trying different things as per the rep online, eventually he gave up and booked a tech to come to the house, at that point I discovered that the wired network still has internet, just the wifi drops.

So I did some checking and narrowed it down to my 3 PCs, if any one of them connect/tries to connect to the wifi, I get a Cannot connect to the Network message and the wifi dies, only a reboot will recover.

Tech was here, tried 10 000 things, no luck and he left.

I did some more digging, so the 3 PCs have a TP-Link T9UH adapter each, I removed them and tried some old adapters I had in a drawer, UNBELIEVABLE it worked, wifi no longer dies.

Here is the thing, all 3 adapters worked 100% until I installed the Xfinity gateway, all 3 now does the same thing,  can't be 3 pieces went faulty in the same way at the same time, I will not believe that EVER!

I am convinced there are settings in the gateway causing this behaviour, but the available settings in this gateway are so limited that it might as well not have a login, where do I look for this?

THANKS!