First, make sure you set the xbox networking to automatic. Then shut them off.
Then open the 4100's web page and got to Basic/Network Settings.
Look at the last section called Number of DCHP Clients. You might see a list of IP addresses for devices you have connected to the router. Select Revoke next to each entry. Find the one that matches your PC. Don't revoke it. If you not the entries for the xboxes, just revoke those. Refresh the page.
After all is cleared out. Turn on one xbox only. Give it a minute. Refresh the routers web page. You should see a new entry listed. This time select the Reserve option to the right for this entry. This will auto fill in the information in the Add DHCP Reservation section. At this time you can give it a different IP address if you wish or keep the current one. Give a name for Computer Name. Something like Xbox1 or LivingRoomXbox, can be anything. Select the SAVE button. This 1st xbox now has a reserved IP address. Simple huh. Do the same for the next xbox.
Remember the IP addresses for each xbox and then you can go back in and Add or Edit the Gamefuel rules so that they are entered in correctly there. If you already have rules set up, just select the first rule and select the Edit option to change the Source IP address information for each xbox. Make sure the names are entered in correctly for each box so you can easily ID each one.
Follow the Sticky from this point on if you need to. I would make sure that Dynamic Fragmentation is off and Firewall settings are set to Endpoint Independent for both TCP and UDP.