I'm using the Comcast gateway in bridge mode to my own router.
Changing the DNS on my router, PC adapter settings, mobile phone, or even through apps and programs that should normally work is having no affect.
My DNS keeps connecting to an IP I don't recognize but is apparently a Comcast DNS server.
I used to run on Cloudflare's 22.214.171.124 / 126.96.36.199 but that's no longer possible.
Nope. That 96.xxx address is not their DNS servers address, it's your WAN / default gateway address. Are you positive that the Comcast gateway device is truly in full bridge mode ?