From reading the other thread it seems that you're unlucky enough to have an Xfinity router (Comcast's own one). Sadly from a little bit of Googling, you can't change the DNS settings on those devices so as to use a PiHole:
There are two ways to go here:
- Buy your own router that is compatible with Comcast's network (I'm in the UK so don't know US systems to advise on this, sorry)
- Buy any cheap router and put the Xfinity router into bridge mode (i.e. make it a fancy modem) and use the replacement router to actually manage your network.
Personally, I'd go with option 1) and get that PoS Xfinity thing out of my network. Just my 2¢!
Whichever option you choose, you will need to know all of your ISP settings to configure the replacement...
Good luck!