Internet won’t let me play online video games

My internet works perfectly fine when getting on calls or surfing the web. My problem is that I can’t play any online video games. Every time I try to, it either a says I’ve lost connection to the games servers or I can’t load into an online game at all. I’ve tried talking to two different Xfinity agents and they have both just told me to get a replacement modem. I’m confident that my modem is not the problem as my modem was working perfectly a few days ago. This all started when I factory reset my router because I forgot my gateways login. Ever since factory resetting it, this problem has been occurring. I’ve tried turning on dmz, port forwarding, restarting my router a few times, and changing some of my routers security settings like turning off its firewall completely and making sure I don’t have any parental controls on and none of that works either. I never even had to rely on those options/settings before factory resetting my router to play online games anyways. I also know that this is a problem with my router and not my desktop specifically because I also can’t play online games on my phone.