The place in Kelsterbach has the massive advantage that it's right next to the train station on the S-Bahn line from Frankfurt (first stop after the Airport on S8 or S9 trains called "Wiesbaden" or "Mainz"). Neu-Iseburg is more of a faff to get to, certainly w/o a car.

Important: You need to use the street address of the post office and the words "poste restante" or "postlagernd". And it's registered mail, you need to use the real name of the recipient, as this will be checked against ID. Ordinary mail can be sent under an assumed name (!), as in:

Enkhamga
poste restante / Postlagernd
Waldstr. 10
65451 Kelsterbach