For a test, does a computer hardwired directly to the router / gateway device with an ethernet cable have the same problem ?
With this test, we are trying to isolate this from being either a WiFi-only problem or a problem with the general connection to the Comcast system which would of course affect both, see ? It's a good first step of troubleshooting before we proceed any further.
Please also post any modem RF error / event log entries.
O/k, they disabled the modem component's RF error logging feature in the Comcast supplied gateway devices.... A BIG mistake IMNSHO !!!! Those router component event log entries are unhelpful, unfortunately...
Have you tried doing that test ?
I wouldn't know what you have.... Sorry. I'm not sitting next to you bud....
Some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods, powerline ethernet adapters, MoCA adapters. Good luck.