This comes from Actiontec website specific for the Verizon MI424WR router:
I was told to change the MTU setting of the Actiontec, so it will work with my Xbox. How do I change the MTU?
There are several problems with this.
1. The MTU setting is not the cause. The default MTU setting for Actiontec routers is 1492 for PPPoE connections, and 1500 for DHCP as well as Static connections. To change the MTU setting, access the My Network icon at the top of the page, then select Network Connections on the left. Then on the Network Connections page select the Broadband Connection or PPPoE Connection that is being used. You can identify the connection being used by checking the Status field, it will be connected. When the Properties page is displayed, click the Settings button. The MTU is set to Automatic by default, but if 'Manual' is selected from the drop-down, a numeric value can be entered into the text box. To save the change, click Apply at the bottom of the page. (But, the MTU setting is rarely if ever the problem.)
2. The actual problem is that the ports used by the Xbox, are not open for incoming traffic. There are two ways this can be changed.
A. Enable UPnP (Universal Plug and Play). Many of our routers and modems are delivered with UPnP disabled for security reasons. But UPnP can easily be enabled. To enable it, browse to the UPnP page in the Advanced Setup. Enable both option check-boxes, and leave the Publication drop-down at its default. Click Apply, to save the change.
B. The other way to open the ports needed by the Xbox, is to use Port Forwarding. The Xbox uses ports 88 UDP and 3074 both UDP and TCP. To open these ports, begin by logging into the router and at the main screen select the Firewall Settings icon (The Security icon in older firmware versions). Make certain that the 'General' Firewall Setting is set to Typical Security or Medium. (Any other Firewall setting will in most cases have a negative effect on the ports that the Xbox needs to work correctly.) From the left-hand menu select Port Forwarding. Click the word ADD (in red) to create a new port forwarding rule. To the right of 'Networked Computer / Device:' select the HOST Name of the Xbox from the drop-down if it is listed. If the Xbox is not listed, enter its IP address into the text box to the right of 'Specify Address'. Then, to the right of 'Protocol' click the drop-down for 'Specify Protocol' and if Xbox is not listed, click the option for 'Show All Services'. Now when you click the drop-down for 'Specify Protocol' there will be many more choices and an entry for Xbox near the bottom of the list. When you select it, the ports and protocols will be displayed. On the 'WAN Connection Type' line click the drop-down and select the WAN connection method that your router is actually using. Typically this will be either 'Broadband Connection (Coax)' or 'Broadband Connection (Ethernet)'. After selecting the correct option click Apply. Now, back at the main Port Forwarding page, click Apply to complete the port forwarding for the Xbox.
http://www.actiontec.com/products/faqs.php?pid=214#q8