I had a server with a reserved IP address. My server died, and the admin page of the xfinity gateway/modem would not allow me to remove the reserved IP address. I solved the limitiation of the admin page by spoofing the mac address on my new linux server. This settings are not kept by you linux and get reset after a reboot.
$ sudo ip link set dev eth0 down $ sudo ip link set dev eth0 address 00:15:a0:ee:c8:9c (use the mac address you want to remove) $ sudo ip link set dev eth0 up That reconnected to the modem and got the reserved IP that I wanted to remove. That allows you to remove the reserved IP by making it use DHCP.
Not sure how to do it on Windows. Here is another option if you don't have a linux server. (I did not try this but I believe it would work just as good) my next option was going to be using an old Netgear router and connecting it to the gateway. Most routers allow you to change the mac address. (xfinity modem---oldRouter---yourComputer)
The admin pages of the xfinity cable modem are "poor"
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