Internet Gateway DHCP/DNS Address Settings

I haven't bothered with the network settings up until now, because I had no problems, but… Today my desktop could NOT connect to several sites and they all showed a version of the "Your connection is not private" or "Can't establish a secure connection to the server" error. It was mostly on sites like Facebook, Twitter, etc., but I knew all those certificates were not expired or forged, and I had no problem on my tablet or phone, just my desktop, and it was on every browser I tried: Safari, Chrome, Firefox, Opera. I deleted caches and cookies and even dumped DNS caches using Terminal, restarted the computer, and the problem persisted. I then went into the network settings and changed from my custom settings to just "Automatic". I don't know if that is what eventually worked, or the other things I did in tandem, but now I see some things I would like some clarity on:

I see now my OpenDNS IP addresses have been changed from 208.67.222.222 · 208.67.220.220, to what seem to be the device settings of: 192.168.12.1 and some long MAC address starting with 2607:fbxxxxxxx, etc. Should I try to go back to my OpenDNS settings, or leave this like it is?

Also, I see that the “Configure IPv6” is now set to “automatically” and before it was set to “Link-local only” Any advice as to what the preferred settings are for these and any other settings in the Network Preferences with this T-Mobile Home Internet gateway?