We had a similar issue at our house. We were using an unmanaged switch and a asus router. Both the router and any windows devices connected via ethernet to our modem would drop internet connection. The devices would still recognise comcast but would not reach out to anything outside of the local network. After going through a couple of different switches I don't think the switch is the issue and it has something to do with the modem itself. Anything on the Xfinity wifi works and now that I have set a static IP for the router it no longer drops connection either which further leads me to believe this is a modem issue.
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