First things first, let's make sure the engine is not seized.
27 mm on a long breaker bar on the crank pulley, Turn Clockwise Only as you are facing the front of the car.

If you can get it to spin by hand then it's an electrical problem.

Get a remote starter switch, go right across the pilot wire and battery terminal on the starter solenoid.

Once you confirm the starter can turn electrically, then go after the starter control chain.

Not much in the path on a 124, it is battery - X4 terminal under the dash - red wire to key switch terminal 30 - violet wire on terminal 50 on the key - NSS - starter solenoid.

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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