Bro, are you sure, without Mobile signals even Sygic or Maps.me dont work? Because I use Sygic in Gujjo, Sind and it worked quite well. The reason is, these apps use GPS instead of internet or Mobile networks. So if Garmin Etrex 10 is working in remote locations, these apps should also work.
Today I extensively used Maps.me and I fall in love with it, than Sygic.
Personally, I would never use Garmin Etrex 10, another device for single purpose is no good.