The traffic going between your client and the chess.com servers has a lot of other connections outside of your local network and ISP. Any problem at any step of the path can cause disconnections, as can processes running on your client that may not actually cause general internet disconnections.
Some things you can look at on your end that may improve things:
https://support.chess.com/customer/en/portal/articles/1444918-my-internet-connection-is-fine---why-am-i-getting-disconnects-
and
https://www.chess.com/blog/News/how-to-adjust-your-live-chess-connection
during last game it says connection failure while my internet connection is fine.......