Your model car should have a 722.6 transmission; you must have the “tool” if you verified the correct fluid level at operating fluid temperature. You have to diagnose from the outside first on any electronic control transmission, any transmission codes.

The early generation 722.6 transmissions had F1 and F2 sprag failure, they were updated, more holding force, less slippage. If you have no gear ratio error codes which usually indicates transmission slippage. I would remove the oil pan check for excessive debris in the fitter and pan. If the fitter is not plugged, the F1, F2 sprags are most likely slipping, internal.