I don't think that that is necessarily the problem. My setup also has my
router set as the primary DNS server but the configuration on the router has
a primary and a secondary DNS configured which would normally be the primary
and secondary DNS of my ISP. If you log onto your router you should be able
to check the configuration and compare it to your ISP DNS server addresses.
I confirmed on my version of Trisquel 8 that “sudo service network-manager
restart” command in the terminal, does restart networking. None of the
changes I made to the DNS servers had any effect until I did a restart, I
suspect that a reboot would have achieved the same result.
I can upload some screenshots of how to change your DNS server addresses and
how to have Trisquel ignore the DNS configuration on your router if that
would help
