Android user here. I clear all running apps a few minutes before attempting to join a BG. (You could even do a reboot 10 minutes or so to make sure nothing is running that may slow you down.) Then I open the
disneyland app and the android clock app. Make sure to have seconds enabled. Clock in foreground, disneyland in background with previously mentioned AT-AT screen ready to go. When clock gets to 6:59:59 switch to disneyland app (can double tap square button on bottom bar to do that) and hit "join boarding group". When I used this technique in the before-times I got a BG each of the three times I tried. And low BG numbers, too.