@GeorgeS93 

The internet connection DNS'es are mandatory for your router to have internet connection (TP-Link cloud connection, updating its FW, time settings, etc.)

If you omit the DHCP DNS'es, the router will use these internet connection DNS'es for all its DHCP client devices - devices use router's LAN IP address as a DNS.

If (for some reason, like a PI-Hole or other internal DNS server) you want your DHCP client devices to have different DNS'es then you can set these DHCP DNS'es different than internet connection DNS'es.

In this case your DHCP client devices will have these DNS'es - not the router LAN IP address.