xfinity keeps forcing its DNS with customer owned modem & router

Router: GT-AX11000 Pro

Modem: Motorola - MB8611

I used this combo for the last couple months and it worked great. I was using google DNS and was seeing major improvements. Then as of a few weeks ago it just stopped holding. Every time I check my DNS is says Comcast Cable and I've seen a performance drop off since then. I know Xfinity forces DNS but I thought if you owned your own hardware you were allowed to use your own DNS? Anyone have ideas 

Thanks :)