Home Networking

First time posting so please let me know if you need more info or anything. I'm trying to figure this out on my own before I call ATT support to make me buy faster speeds or something crazy. But anyways, I currently pay for ATT Fiber 500 MBPS, using their provided Gateway (it's to my understanding third-party hardware would not work with their network). All of the cables and wiring are less than 2 years old as we had to pay ATT for a completely new set up. So, we are the first users in this house. About a month ago, we installed Ethernet cables for the other two PCs in the house, and ever since then all of the other devices will either have very slow internet intermittently, or completely lose internet. This is especially noticeable every single time I use my PC, as it will lose internet at least 5-10 times within the first 10-20 minutes of start up, and be told in video games I have packet loss (but moderate ping).. I believe I used to run anywhere from 6-12 ping, now I'm at 20-30+... Despite being on the other side of the wall (literally) to the gateway/router. I'm trying to not have to install yet another Ethernet port for my PC just to play games (and host Baldurs Gate runs without it DC'ing everyone), but also not ruin the Ethernet users' speeds. Approximate total devices on the network are 3 PCs (two ethernet connected), 3 cell phones, smart thermostat, security system and peripherals, 2 Rokus, 3 pet trackers, robot vacuum, then variety if the devices are powered on or not includes a tablet, Nintendo Switch, VR headset, and probably a couple other things I can't think of right now. My current solutions ideas are to throttle the ethernet ports (last I looked the speed options were not many...), or to try and set up my range extender as an access point and put all of the security system and perhaps a few lesser used devices on it, but that would also be connected to an Ethernet port so perhaps that would make things worse.