@girish I have a similar issue so instead of starting a new thread I thought I'll post here. My VM running cloudron is unable to resolve nameservers since installing unbound on my router. Came across this thread and added the below lines to /etc/unbound/unbound.conf.d/custom.conf
It worked for few days and broke again today, access to cloudron still works but when logged on to the VM it cannot resolve DNS queries, hope it makes sense. Any ideas why DNS resolution works intermittently?