OK, but it wouldn't save the port forward until I changed the deco XE75 to a static IP. I was choosing the device from the 'Use Client IP Address' option which was already reserved in the deco XE75,...

The version on the bottom of the deco is 2.6, though that wasn't an option for the hardware version. When trying to add a port forwarding rule via the app->home->more->Advanced->NAT Forwarding->Port...