As you already discovered 10.0.18.1 is not routable private IP. You ISP (Comcast) does not put cable customers behind GNAT, I believe - so, as already mentioned above it is likely openSuse not your ISP "feature".
If you run out of ideas, and following the comments in resolv.conf didn't get you anywhere - you could try to fish for leads by: sudo grep -r 10.0.18.1 /etc This will likely lead you to a systemd service and/or network manager config files, depending which one is managing your DNS config. I am less and less familiar with openSuse these days, moving slowly away to different distro(s), after decades or use.... Suse is becoming pretty nasty shop in my personal experience. They are unable to provide even the most basic billing support for paying individual customers, blocking emails due to broken infrastructure and abandonment, not even answering their corporate phone numbers, ....., leading to completely unnecessary annual legal credit card billing disputes. Hope that helps, Tomas
