1) Are you sure it wasn't stuck in 2nd (the dreaded limp home mode)?
(Was the Check Engine light and Check Engine Electronics shown in the display?)
Get an OBDII reader or go to the Auto Parts store and get them to pull codes.

2) As you noticed there is no transmission dipstick, only a cap with a seal. There is a dipstick tool available that can be used to check the fluid level.
http://www.pelicanparts.com/catalog/SuperCat/3742/MBZ_3742_TOLTOL_pg11.htm# scroll down to the bottom of the page

3) Pelican Technical Article: Mercedes Benz - Automatic Transmission Fluid Change

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech