Firewall: not a valid ip address - RT-AC88u

[ThaDoc]

Hi!
I received my Asus RT-AC88u today. Configuring the firewall won't work. When I try to add a port to my NAS a pop-up window opens: "192.168.1.200 is not a valid IP address!". I tried entering the IP address of my PC instead with the same result.
My router's IP is 192.168.1.1 so it's in the same range. Anyone else experiencing the same?
Edit: I'm on firmware 3.0.0.4.380_1354

 

[ThaDoc]

Ok, I'm dumb...
It says: IPv6 firewall... and I'm trying to enter an IPv4 address. Where's the firewall settings for IPv4?

 

[ThaDoc]

Found what I was looking for: WAN > Port Forwarding.