I would politely reply “there is nothing wrong with your modem, I don’t want you spying on my Internet activity.”
My ISP isn’t nearly as intrusive as that, but they do some pretty shady stuff. As far as they can see, I’ve only one device connected to them (my pfSense firewall) and my DNS traffic is encrypted (DoT) so all they see is IP addresses (which frankly is enough for them to track me, they are just lazy) and mostly encrypted traffic. And I’ve strongly considered setting up a VPN, but that just moves the “spies” to the VPN provider rather than the ISP. Though there are ways to set something up in one of the cloud services to have your own private VPN, but it’s still running on “other people’s computers”.
Many ISPs provide a combo model/gateway router and call it the “modem”. But it does both.
Your router, not one provided by the ISP? Then there shouldn’t be anything that the ISP does that breaks your LAN because that would all be configured and controlled by your own router. Unless you are setting it up as a dumb AP in which case not only are you getting all your IPs from your ISP, but all your devices are directly accessible from the Internet without protection.
Why? If you configure the router with the same SSID and password than all of your wireless devices will be able to just connect to the new router with no changes. I used to have to do this all the time.
I think there is some key detail or piece of information here that in not clear because I’m not sure how this problem manifests. It doesn’t really matter though, you’re up and running now if I understand correctly.
Hello fellow Xfinity customer. I wish I had any other choice.