If its a Merc box check the oil levelFind a good local autobox specialist.
Sounds like you have a valve failure in the shift system. the old adage applies "no user servicable parts inside"
PS IIRC it's a Mercedes autobox.
which you cant do according to merc as there is no dipstick.
except you can buy one ( just a flexi cable) from fleabay for a few quid.
into the little tube thing with the yellow cap ( dont do this if its under warranty as it has a paper seal), pull it out.
if its short of oil, it is short of oil.
if there is oil see if its discoloured. Sniff it. If it is remotely manky or smells a bit you have a band problem
also check any plugs and connections. A poor connection will cause issues. If you can get the gearbox ecu read that will identify any areas of concern.