By default DNS uses UDP/53 not TCP. Does your firewall allows DNS queries from outside? Actually it does, if it works on WIN10. There must be issue with DDWRT setup, Hows your OVPNs IP Network described? Do you have DNS Servers in PPP Profile?
Well, i doubt its issue on MT side. I do not have a way to test whats with DDWRT settings, but can you point ( just for testing purposes) DNS servers of DDWRT to mikrotik and then test resolving,
Do you have some DNS Proxy on DDWRT or some catch-all-DNS firewall rule, or some similar mechanism?