I’ve been having issues with /etc/resolv.conf resetting after a reboot or periodically. I set static IPs on the NICs but this wasn’t enough.
To fix it I:
nano /etc/resolvconf/resolv.conf.d/base
And put the relevant settings, e.g.
nameserver 192.168.1.XXX
Then, regenerate the resolv.conf file
resolvconf -u