Hello @uEUcLnQf . My apologies you are experiencing this issue. As long as you have a strong password, with Xfinity xFi, your network should be secured. Please make sure that these devices that keep connecting to your network are devices that should be connected (ex : smartphone(s), tablets, smart tvs, wireless printer, or the gateway itself). Also, would you kindly provide answers to the below questions.
Their Advanced Security feature has recently been pushed to their gateway devices via a firmware update whether you want it or not, hence the hypochondriac notices. It's buggy.
We've seen many many posts about this here lately ! The work-around is to disable the Advanced Security and not use it for now, it's buggy.;