We’ve received reports from some users about not being able to use the matchmaking feature to successfully enter dungeons, battlefields, arena, etc. If you are experiencing this issue, please try one of the following solutions.
1. You will need to reset your MAC addresses within your console when facing this issue. A Mac Address is a "Media Access Control Address". It is basically is a number attached to each hardware which in this case is an Xbox. It allows for authentication of a specific Xbox number to connect to a network. The mac address can be seen as a ticket to enter through a gate. It happens in other games as well with server connection issues.
Instructions: Press Home Xbox >> System Tab >> Settings >> General Tab >> Network Settings >> Advanced Settings >> Alternate MAC address >> Clear
2. Perform a hard reset to your Xbox. This can be done by pushing down on your Xbox One's power button until it turns off, then starting it again.
We appreciate your support and patience in this matter.