C7500 internet speed constantly slowing down

Title described everything. For the past two days, I've had issues where I was getting over 100+ MPBS on my connection as normal, and then it goes down to 2.5 MBPS or even slower. The connection will not stay stable, and I am constantly getting slow internet speeds.

I called my ISP Xfinity and they troubleshooted everything. They tried modem/network resets, and sent a tech out. The tech confirms the cable connection is good and is giving a solid speed from the cable line, and they have diagnosed it as a modem/router issue on my end.

My router was purchased over 1 year and two months ago from Amazon, and it's out of warranty and support.

I need to know if there are any troubleshooting tricks, or if this is a hardware failure and I need to buy a brand new router.

If I have to buy a brand new router, it won't be this one again, cause I've never had a router fail after one year of purchase, especially since I spent over $250 on this one since "It was top of the line"