I believe it is region locked. As soon as my xbox swapped from my wife and me, to just me and lobbied with a bloke from the USA we found the match instantly.
Shoutout to the legends that helped me with that, you know who you are
Been a while, but if memory serves me, we had good success by doing the following (assuming you have enough people/accounts for 2 full groups):
Get 2 people who are closest geographically to be the team hosts. 1 should be on a box by themselves, the other should be playing splitscreen. Have the one playing on a box alone search for a match. Have the splitscreen host wait 5 seconds, then search as well. You should get matched up.
Splitscreen group leaders can’t be game hosts, so this forces the one who is a team host on a box alone to be game host, then the splitscreen team will find them easily.