Why do we even set the DNS servers on the rooftop WISP radio?

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On my rooftop WISP radio, the DNS Servers are set to the  
typical Google Servers, and

However, I have been testing for DNS leaks at https://ipleak.net
and I definitely have a DNS leak even when I'm on VPN!

In testing, I noticed that both my home wifi router & the rooftop
WISP radio had the same two DNS Servers set up, so, I changed the
two servers on the home broadband router to a set that I found here:

What I noticed is that the DNS server used was never the DNS  
servers set up in the rooftop WISP radio!

The DNS servers that show up in the tests at https://ipleak.net
and at https://www.dnsleaktest.com were ONLY the DNS servers set  
up in the home broadband wifi router.

Given that the home broadband router is "winning", I tried to  
put an incorrect DNS in my home broadband router, to see if
the rooftop radio DNS settings would act as a "failsafe".


I was *assuming* that the rooftop radio's DNS servers would win.

But, the rooftop radio's DNS servers never kicked in and I  
temporarily lost my ability to ping by domain name when I put  
in a bad set of DNS servers in the home wifi router.

What is going on?

Specifically, why would the rooftop radio even *have* a setting
for the DNS servers if the setting is never used?

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