Thanks for the excellent information. Yes, that worked but now I'm wondering whether I can comment all the lines out except one chosen IP address so that I don't need to be concerned about the IP address changing randomly. Does this make any sense?
You can but there is no real reason to do so. If that resolver goes down you are left without DNS.
A better (and safer) solution might be to run your own DNS resolver on the VPN server, optionally with intercept rules so that you could block unwanted traffic on DNS level.