As docbert pointed out, for Android it's based on your Google Account country, which is difficult to change but you can just make an extra account. The downside is that you don't get updates unless the spare account stays logged in.

For iOS, you can change your App Store country in your phone's settings--it's a different setting than the 'region' of your iPhone, and they don't have to match. The page says it requires a payment method but you can often just fill in an address in that country and pick 'none' for the payment method. Installed apps will update even if you've changed your country to one where that app isn't supported.

Most of the geofencing seems to be for security purposes with finance apps or to maintain regionalization of the app, e.g. Domino's Pizza has a completely different app in the U.S. vs. India so they can reduce annoyance from downloading the wrong one by geofencing the app.