I have had the Mifi 4510L for a few months now and when I bought this product I was told that even though my area only supports 3G at the moment that I should have no problem connecting to Xbox Live and playing.
My first problem is the NAT setting, I am unable to get into games because of my NAT is set to moderate. My second problem is that I can get into a game and play if someone invites me to a game instead of searching but after a few minutes of playing the Mifi disconnects me from Xbox Live. So, if I do get into a game I have a few minutes of playtime then I'm kicked out. Is there any way of solving these issues?
I wonder if you are referring to some different kind of NAT when you say the NAT setting is getting in the way of your xbox connection. The MiFi 4510L does not have a NAT setting that I know of. NAT is a networking technique where one IP Address is used for multiple computers by adding a port number to the end of a IP Address. In any case you cannot change this setting nor should you have to worry about it. The MiFi does have a port forwarding area which could be helpful to allow you to host games, but I have no experience setting that feature up to work with xbox.
You second issue mentions getting disconnected. When you are disconnected are you kicked off of the MiFi completely? I'd be willing to bet that if you turn off the Auto Connect feature on your Xbox that you would have to sign in all over again. You might want to try switching the channel which your MiFi operates on to make sure there is no interference. You could also try lowering your 802.11 mode from g/b/n to g/b. If you enable the logging feature on your MiFi you might be able to see evidence that your MiFi is dropping calls while your Xbox is in use.
Let me know if you need the specific steps to any of these suggestions.