It is unclear to me what is wrong precisely without being able to run some code, but there are a few things to consider:
1. QNetworkManager requires a Qt event loop to be running to function properly. Is that the case in your DLL? (In any case this should not prevent get() from returning.)
2. QNetworkManager may require QCoreApplication to be instanciated, although I could not find anything supporting that intuition in the docs.
3. Are you certain that doCallback("before get: " + remotePath) returns? It might have time to print something and then freeze before returning.
Can you reproduce the problem in a minimal example?