I am not very techie but very quickly learning to be. My son had an XBOX ONE for Xmas previously used XBOX 360. He's struggling to play online with friends. I understand he needs NAT TYPE Open tried opening the relevant ports twice one of which no 53 says its reserved so can't open it. Spent an hour on the phone with a very helpful guy at Virgin assining a permanent IP address for the console and still it shows as Strict really don't know what else to do. Microsoft says to reset console to factory settings which distresses my son !
You mention giving the Xbox a static IP address, had you also assigned that address to the DMZ in the hub? This will negate any port forwarding that you may need. Also is the xbox connected via ethernet or wireless?
I've seen numerous reports of customers having trouble with Strict NAT due to the xbox being set to 'instant on' You can change this in the xboxes Power options.
You're right you can only have one device in the DMZ at a time but you can set port forwarding for the xbox which will act similarly. I won't know which ports in particular you'll need but a quick google should help.
Here's a simple guide to port forwarding on the hub should you need it.
Let us know how you get on with the power options on the xbox.