Hi, @I am Deathclaw
You are having trouble joining our regional leaderboards because your gamertag is invalid since you changed it. You will need to update it to your new one which you can do here.
Once you've done that, please head to your stats page and collect your stats to ensure it's working. If it's working with no errors displayed, please see the next step otherwise please post here letting me know what error you're getting when changing your gamertag.
To add yourself to our regional leaderboards please go here.
As soon as you load the page, the system will start guessing where you are based on your IP. If the results are incorrect, click the blue 'Yes' button to enter your location yourself then click the green save button. Now go back to your stats page and collect your stats to ensure the system is updated.
That's it but if you have any issues, just let me know.