Turn by Turn notification relies on the underlying mapping data, if the necessary data hasn’t been added to the map, or what has been added is incorrect, then no app or device can give you that info.
TbT is a “shortest path” algorithm, as such it works on junctions to let you know when to turn. Obviously this is fairly straightforward when you are on roads but not so much off-road. A track on the ground might not be a legal RoW, it might just be an animal track or a new farm or forestry track, so what you see as a junction of tracks on the ground is marked on the map as say a left bend in the one track it knows about.
Basically you can’t just turn your brain off and expect a device to guide you along the route. A bit of map, paper or electronic, reading knowledge and basic navigation skills are still needed: “along this track, over the hill and 400m later turn left then take the second track on the right” type thing so you aren’t constantly referring to the device.
For local road rides (and some local off-road) I’ll use a Garmin 520 as I know the roads and trails. When I say local, that’s basically the Yorkshire Dales and the Lakes. For areas I don’t know then I’ve a Garmin Oregon with both OS and OSM maps