High Ping in Games

I need some help figuring out what is wrong with my internet. I am getting very high ping in games such as League of Legends, and Call of Duty: Modern Warfare on PC, making them near unplayable.

My ping jumps up and down from anywhere between 90ms and 300ms every second., and spikes around 500-700ms every 5 seconds or so. I am using a hardwired connection, but the same ping issue occurs across multiple devices regardless of whether I am using wifi or hardwired and regardless of how many devices are using the internet at the same time.

I've been trying to troubleshoot it as per instructions on this forum and Xfinity support. The forums reccomended removing the cable splitter, but doing this didn't solve the problem. Xfinity support reccomended I buy a new router so I bought a brand new onw for $300 and still experienced the lag spikes so I returned it.

For some reason, about a month ago the issue went away and my ping stabilized around 20ms in most games, but the issue returned two weeks ago and I'm back to getting destroyed in every game I play.