How to remove data cap for xfinity home internet.

I’m looking to remove my data cap, but every time I go to try to the “use own device” option next is greyed out unless I select an option with a modem and router combo.  I don’t want or need one I have one that’s been working fine for years with xfinity and I have port forwarding and Mac filtering set up that I don’t want to redo and probably can’t do on an xfinity owned device.  I don’t even know why they have data caps no competitors do.  Both ATT and Astound in my area are a little cheaper and no data caps.  If I can’t get unlimited data without an xfinity modem I will be switching.