There is a work around for this problem by disabling IPv6 in the fritz box interface.
Please disable it by going to Internet > Account Information > IPv6 > Untick ‘IPv6 support enabled’
If that fails to work please follow the below to disable teredo filter;
On the user interface of the FRITZ! Box “Internet”.
On the menu, “Internet” on “filter”.
Click on the “Listen” tab.
Uncheck “Teredo Filter active”.
Click Apply to save the settings, click “Apply”.
If the xbox is still showing Strict/Moderate NAT then the only option is to place the console on the DMZ described below:
Click “Internet” in the FRITZ!Box user interface.
Click “Permit Access” in the “Internet” menu.
Click the “Port Forwarding” tab.
Click “New Port Forwarding”.
Select the “Exposed Host” entry from the drop-down menu “Port forwarding enabled for”.
Select the network device (for example a computer) for which you wish to enable port forwarding:
If the IP address is dynamically assigned to the network device by the FRITZ!Box’s DHCP server:
Select the (computer) name of the network device from the drop-down menu “to computer”.
If the network device has a static IP address:
Select “Enter the IP address manually” from the drop-down menu “to computer”.
Enter the IP address of the network device.