Router gets VERY slow and needs reboot

I've had this router for about 1 year. It worked well, but after about 3 weeks things would start streaming slow, movies and games. After a restart everything worked fine.

A month ago I started having issues with my wife's phone, Galaxy S6. It would stay connected, but internet was so slow it was unusable. The mobile network was fine. If we connected to wifi at a store or libray it was fine. Only time we had an issue is at home. To give you an idea if we did a Speedtest it would take 1-2 minutes to "Find the Best Server". After that push "Begin Test" and the speed was about 75 Mbps. For my phone the "Find the Best Server" was 2-3 seconds. Do the same test on the mobile network and it works great.

I tried moving it from 5 ghz to 2.4, changed the encryption, rebooted the phone, reset the phone, rebooted the router nothing helped the speed. Finally I tried connecting with WPS instead of using the passphrase and that worked. For a while. Every 1-2 days it gets slow again. I need to reboot the router, then it works fine again for another 1-2 days.

Provider is Xfinity and I believe we have the latest firmware.

Needless to say it's very frustrating to have to reboot the router every day. Today it worked only 12 hours and I had to reboot again. We have 4 computers, Chromecast, smart appliances, ipods, and none of them have a problem. Only 1 phone and then it only started a month ago.

Any ideas? This is very frustrating.

Thanks.