I'm not to experienced in Network setting for Xbox, but this is what I've found from a valuable source:
Originally posted by another user
MTU The Xbox Live service requires your home network have a certain Maximum Transmission Unit (MTU). While this technical detail can normally be ignored in home networking, MTU values are important to the performance of online games. If this test fails, you can adjust the MTU setting on your network router or equivalent device to resolve the problem.
More (xbox.com)
When I had network problems, I just followed the unique guide on Xbox.com and I advise you to as well, its really helpful heres the link to it:
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Hope this has helped.