I agree with @HDCerberus and kudos for doing all that work to figure out what was going on.
I’m not convinced that all routers and Operating Systems are clear on what they do with DNS fields. I’ve selected my own DNS settings in Windows and pcWRT. Using ‘nslookup google.com’ from the Windows Powershell CLI shows me that Windows is consistently picking the first DNS entry. Here’s an example where the result shows CloudFlare’s server and DNS address.
PS C:\Users\user> nslookup google.com
Server: one.one.one.one
Address: 1.1.1.1
Regardless, the advantage of an agile company like bigXi LLC is that they can be clear on their implementation with a single line of text near the DNS setting, that indicates their approach is to use load-balancing. I’ve also noticed that they sometimes use an information icon next to some fields which could be used instead of a line of text.
It would also be helpful to have a note when setting the DNS in pcWRT that the DNS settings in the ‘Settings>Internet’ page will be ignored if ‘Access Control’ is enabled. (I believe that is the case, but others can confirm.)