How can I use existing gps routes on my phone?

[Wilfried]

Do you have a solution to this problem or a suggestion for another program/app?

I use an app called Locus (https://www.locusmap.eu/). The main reason for me was its capability to create track logs (instead or in addition to a dedicated GPS logger), but you can also import existing tracks, you created or modified with RoutConverter. You can even create or modify tracks with Locus on your smartphone, but due to the small display, it is much more convenient to use RouteConverter on the PC and transfer the gpx to the phone.

Locus can also use offline maps, which you download from several sources.