The problem is how to set the dns server address as to be assigned to all the authenticated hosts.
I was able to set the DNS server address in the domain configuration. Please note that this needs to be the domain used for L2TP authentication so the same one you bind to the L2TP group. The command sequence to set the DNS server address is as follows:
ip pool 1 10.0.0.2 10.0.0.100
referenced by user vpdnuser
dns primary-ip X.X.X.X
dns second-ip Y.Y.Y.Y