It it becomes too much of a nuisance for you, you can buy a 2nd router (even an inexpensive one), place the Xfinity router into bridge mode, and then use the 2nd router for your network.
In the above setup, the Xfinity router will act as a modem only. No matter how many disconnections you get, your 2nd router's setting will be unaffected. (You will likely get no disconnections since the Xfinity router is now in a modem only mode.)
Alternatively, you can also just buy an Xfinity compatible router and use that as your primary router and return Xfinity's router to them. The ISP will be prevented from making changes to routers not owned by/leased from them.
There are some reports that the ISPs keep resetting their customer's routers to the g/n setting. Some have even gone to the extent of preventing this mode/protocol change on some of their router models as well.
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