When I go to play one certain game with friends it says can’t connect with them due to MTU...

MTU isn't a Xbox setting, it's just a requirement for it to maintain a certain certain packet size, it helps maintain the quality of your gaming connection.

Most commonly with DSL it's going to be an issue with a filter on the line, the gateway they gave you starting to go out or in some cases it's also what pops up when your connection is being throttled for billing issues with the ISP.

Though the biggest problem with testing that is mainly that DSL companies don't hand out modems, they give gateways what are demi-routers. Even if there is only one port on the gateway it still has firmware on it that can have issues like any router and
doesn't in that case fully test if it's a line issue or not by plugging directly in.

What did your ISP say when you called them?

we would need to call your Internet provider

No just re download them

Certain games will not play in other regions like upcoming titan fall

Due to technical issues like connectivity but that is due more with non supported countries

When I invite someone to party or join someone it says connecting and then it’s will say party chat disconnected and then I can’t join any of my friends does any one know how to fix?

You can always opt back in to load the worlds again but I can’t say for certain whether or not you will be able to play them when the update rolls out, my guess would be yes but thats just my guess.