Custom subnet

Hello,

I was trying to setup my second router to be part of the same network as my Helix Router but when I tried entering the subnet 255.255.0.0, it does not let me.

I can only choose either 255.255.255.0 or 255.255.255.128 which does not work for me.

Is there a way I can choose my own subnet mask?  Via SSH or telnet maybe?

Thanks