I'm confused at the scenario you are presenting. So in this case, you travel out of the EU, but are installing a taxi app from a non-eu country, but the app is only available in the EU?
Let's get to a more down to earth example then: you break your phone while abroad, you can't buy a new one and reinstall old apps anymore.
Or another one: you forgot to install an application available only on this alternative store before leaving (it's not that crazy, see F-Droid on Android where apps are not always available on the Play Store), you can't install it anymore since you aren't physically in the EU anymore.