Sorry, but on the 210 car with a 606 diesel, there is no fan lock
hole like on a 300e or other to hold the pulley.
You have to use a pulley holding tool that grabs the pulley and belt and uses the belt to hold the pulley with leverage.
The P/N is 603 589 00 40 00. The standard old 300E 8mm
fan bolt wrench works fine for breaking the bolt loose.
I have seen people (not here) use vise grips or channellocks
to grip the lip of the pulley from the rear, but I cannot recommend this as it will damage your pulley.

Good luck.
Dr.D