It sounds like you may have an improper vent connection OR the installer may have made a common mistake of forgetting to trap this fixture thinking the sump tank could act as a trap. Not only should the fixture be vented (NOT pro vent as it is not a professional job only a low life would use an AAV) But the Sump tank should also be vented.
The stench your smelling could very easily cause you a severe health hazard and not be taken lightly. Good luck