What region is your Mi-Box set to and are you using the local App and not the US/UK App?
Hmm, there is definitely only one Disney+ app available. How can you tell the difference?
I'm assuming this is most likely the problem, cause there is no way to log in with my number (as with every other device).
After the "Not available in your region" screen, there is a login option, which only accepts an email login. And the email I'm registered with says it doesn't exist.
It makes sense now, but it's strange that my Google Play store only gives me access the international localisation? Even though I'm pretty sure everything is set to SA region.
Edit: It turns out the Google account itself was set to UK for some reason, so it forced only the UK play store, even though the region on the device was SA.
Well, problem solved. Thanks for the help! I wouldn't have figured it out without this clue.