Have the n+ wireless router/modem trying to connect via a cable to xbox live but keep gettin insufficient MTU even though the actual MTU of the router is higher than required

You may have to use trial and error to find the best MTU fit:

MTU Default of 1500 to 1492
Tart the setting the MTU @ 1400

MTU setting. Xbox LIVE requires a minimum MTU setting of 1364.
The information your console sends and receives is broken up into "packets" for transmission over your network. MTU is a setting that controls the size of these packets. If it's too small, you won't be able to connect to Xbox LIVE.
Top Solutions Try these solutions in order. If you give each one a shot and you still can't connect, contact Xbox Customer Support. Note that they will ask you try the suggestions on this page first.
Solution 1: Check your router MTU settings Xbox LIVE requires a minimum MTU setting of 1364. Usually, you can change your MTU setting from your router configuration web page, which is supplied by your router manufacturer.
Learn how to find and use your router configuration page
Look for the MTU setting on the router configuration page. When you find it, set it to Auto or 1364 (or higher), save your settings, and then try to connect to Xbox LIVE again on your console.

Solution 2: Connect directly to the modem If your router's MTU is set correctly, try bypassing it by connecting your console directly to the modem. This will help determine whether there's a problem with your router or something else on your network.
Note
This may require you to move your console to a location close to your modem. This is a temporary arrangement, meant only to troubleshoot your connection. If your console is connected wirelessly, you'll need an Ethernet cable for this step.
To connect your console directly to your modem (wired connection):

1. Find the Ethernet cable that runs between your console and the router.
2. Unplug the cable from the router, leaving it connected to the console.
3. Find the Ethernet cable that runs between the router and the modem.
4. Unplug the cable from the modem, and plug the cable from your console in its place.

1. Unplug the USB connector for your adapter from the console. The adapter can remain attached to the console.
2. Plug an Ethernet cable into the Ethernet port on the back of the console.
3. Find the Ethernet cable that runs between your router and the modem.
4. Unplug the cable from the modem, and plug the cable from your console in its place.