Firmware suggests for AC1900 C7000

I have a C7000 that I'm wondering if I could put to use as a plain router behind the ISP's cable modem.

My ISP will not let me use my own cable modem so I can't just plug it into the cable. The C7000's stock firmware doesn't let me disable the on board modem and doesn't seem to want to go very far into the setup without hooking it up to a cable.

I poked around DD-WRT since that is what I use at home and it does support the AC1900 R7000 which is what the C7000 is based on, but there were also a few "I wouldn't try it"s when the question of installing the R7000 version firmware on the C7000 (though no reasons were given why).

Is this a bad idea still? Or is there another firmware out there that anyone has experence with working on the C7000?

Thanks!