I am having the same problem for months now with our Comcast Internet Service. Happens most days generally starting mid-morning through late afternoon. Including weekends. Despite having 250 Mbps down and 12 Mbps up we might as well not have internet at all during these periods.
When this issue is happening we turn off the WiFi on our phones to get data through the cell network.
Our desktop computers and notebooks can only access the internet very sporadically during these periods.
However, when I am connected to the Internet via the cell network the Xfinity app on my phone reports all is well with the Internet while this is going on. Depends on your point of view perhaps.
The most recent firmware update Comcast sent a couple weeks ago to my Arris SB6190 modem (FW version is now: 9.1.103AA40) messed up the activity log resulting in being able to view the log only by reading the html format. Also, the log only records the 10 most recent events so getting a longer term view of the issue is not easy.
The modem log shows sync issues, partial service, timeout issues, no ranging response, etc.
Unplugging and re-plugging the modem changes nothing.