I wouldn't bother with an app that is not easy to use
The people developing the Trans Europe Trail have asked that the beta type route data that has been circulating to date should not be used until they have made their finalised version (test versions have been in use by some folk who have been asked to do just that, test the route data).
For an app that handles gpx files; there are a few such as backcountrynavigator or viewranger.
Personally, I have never bothered to do suchlike, but if I did want to then the nav app that I use has online directions:-
https://www.mapfactor.com/en/support...rt-gpx-tracks/
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Dave
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