@snoobieb
Hi,
So if the Xbox connects, Voice chat works and other games are working it not likely an issue with the Xbox or port forwarding for the Xbox. Likely the issue is the game itself is in need of specific ports to be opened.
You can try one of two things:
1. under Advaced - Nat Forwarding - DMZ you can enable the DMZ to the X-Box address. This will open all ports for the Game Console and in turn all services and games servers it connects to. You will want to asign the X-Box a static IP address that is outside of your DHCP address list by default the address list on the CR1900 is 192.168.0.100 to 192.168.0.199. So a static IP could be 192.168.0.1 to 192.168.0.99 or 192.168.0.200 to 192.168.0.254. The reason for this is each time your DHCP address changes you have to change your DMZ address.
2. Find out which ports Fortnite needs to be open and set up Port Forwarding rules under Advanced - Nat Forwarding - Virtual Servers. Again it is recommended to setup the X-Box on a static IP if you do this.
Best Regards