Frank's place is perfect! It's well connected with a S, U and M transportation 1 min from the apartment.
I was traveling with my elderly father and having transportation everywhere was really helpful (but checking the map, you can walk to a lot of places too!).
The area is great, with plenty of food options and shops, and close to Mauerpark and its flea market on Sundays.
Frank also accommodated my request to put an extra mattress for me to sleep on, as my father took the bed.
The apartment was clean and tidy, and the shower is very spacious.
He's great at communication, quick to reply, and sends detailed instructions for check in and check out.
Totally recommend it, and would stay again if I visit Berlin another time!