Hi,
The AssetTracker library on GitHub was last updated Feb 21. I think that means the errors identified in this thread Asset Tracker erratic haven’t been fixed.
If so, the GPS library being used seems to be somewhat fragile. It might not take much of a reduction in how often it’s called to to cause it to never correctly parse a fix.
So if there’s stuff going on in your loop that limits much at all how often transmitirGPS is called, that could possibly the the problem.
I use a different library (TinyGPS++) in an app that polls other sensors and is successfully running with SYSTEM_THREAD(ENABLED). But - one of the biggest timing issues I had was ensuring that the round-robin of polling got back to the GPS quite frequently. This is necessary unless you park in a tight loop reading serial until you get a valid GPS sentence and only then go do other processing.
HTH, Mike