Port forwarding using ip adress - Helix Fi

Hi,

I'm trying to portforward a port to a server running linux. I know for a fact the device is connected to the internet via my helix router. However, it doesnt show up in the list of connected device on the app. 

Therefore when i try to create a port forwarding rule my server doesnt show in the possible devices to forward to. I, very annoyingly, cant just enter the ip adress to port forward to. Is there a workaround to this badly designed system? Or is there a way to use an ip adress when setting up port forwarding?

Edit: 

The server doesnt use an IPv6 adress.

I can ping the gateway from the server without any packet loss