@clemsongt Currently I can't see any method to let VPN clients use the local DNS server.

We can only modify the WAN settings DNS and from my test the VPN clients will use this DNS server. However it doesn't make sense if your DNS server is in the local. 

Somehow I think it may work:

If your tplink router is connected to another Modem Router, and gets local IP address as WAN IP. Then you can link the DNS server to the Modem, so the DNS server and tplink router WAN IP are on the same subnet and can communicate.

Then set up the DNS server IP on the tplink router WAN port.

I didn't test it, just answer by guess.