Cannot see downloaded .gpx file in offline maps

Hi, I have downloaded a .gpx map from sunshinebooks.co.uk. The map was in a zip folder which I unzipped on my smartphone (MotoG6). I moved the unzipped .gpx file to Locus/maps but it does not appear on the offline maps list. Using file manager on the phone shows the file in Locus/maps folder with other maps I have downloaded from the 'online' tab. This is the first .gpx file I have tried to download. Any ideas how to make this work? I have tried renaming the file as .osm.map as per a suggestion to someone but this does not work. I notice that the maps I can see have a file type .sqlitedb-shm or .sqlite-wal. I have tried changing the name to this type and the map still does not appear in the list. I notice that in file manager on the phone it shows the downloaded locus map files folder as Internal storage/Locus/maps, whereas when in Locus in the maps folder the file path is shown as /sdcard/Locus/maps but I don't have an SD card in the phone! I can still open the .sqlite files though? Thanks