XR300 connection issues over ethernet

Hello, I am new to this forum, but I felt the need to seek out some help with my router. I am currently with AT&T Fiber via U-verse. I have a Pace 5268AC with my XR300 router(DMZ+, what I read was the correct way) and it was working fine for a year minus my hiccups with Blizzard games after playing for 2 hours I will get disconnected, but recently it's just been acting up. I have reset the modem and router and did everything fresh and the issues persist. And those issues are I cannot start an Xbox game bar party chat when I am hardwired into the XR300 but this issue does not exist when I am hardwired into the 5268AC. same thing with playing WoW, I will get rapidly disconnected when I am hooked up to the XR300 but not the Pace. I honestly have no idea what to do because it seems to be fine for Wifi just not hardwire. any help here would be most appreciated thank you.