Unfortunately, I did not understand your problem. However, you can use the normal Nextcloud Android app, synchronise the folder with the GPX files and then open the GPX file in the Nextcloud Android app via Share, e.g. share with OsmAnd.
As alternatives to OsmAnd, take a look at Organic Maps and Magic Earth. Organic Maps can display tracks but cannot route by them. For cyclists but at least for hikers this can be sufficient. Magic Earth can route by tracks, but is a little difficult to use. There is a little trick for this. I personally find Magic Earth better for motorist routing.