New fiber and latency issues.

A few days ago, we got AT&T's fiber internet. It's new in our area and can only get 18Mbps which is fine because we will upgrade as the higher speeds become available. There are 3 computers and 1 PS4 connected through an ethernet cable to the modem/router. The only problem we've had is on my computer - the second one to be connected if that matters. The first pc and mine play World of Warcraft, but mine is having huge latency issues. I can log on at ~70ms and within a few minutes I'm at best 400-600ms but pretty consistently 900-2400ms. I've tried unplugging/plugging back in the ethernet cord. I've changed dns settings, ipconfig/release, renew, and flushdns. I've deleted Cache & WTF folders in the WoW directory. I've restarted, run diagnostics, and searched for solutions to no avail. The only change was plugging my pc from our old modem to the AT&T one. Can anyone shed any light on this issue? I feel like I'm the only one since it's only my pc that's having any issues.