Changing DNS Server

I removed the password with this: Reset Foris password and did set a new one.

In the Foris Web Interface I can only set predefined ones? How can I set custom ones? I am running two DNS Servers in our environment which I want to use for the turris omnia as well.

EDIT: Ok I tried to set the DNS Servers in WAN DNS Server 1 and 2, and then in DNS “Use forwarding” yes, and DNS Forwarder “Use provider’s DNS resolver”, but it does not seem to work, I cannot resolve anything from my local network.