I actually think there is a Bus which goes from the airport to Kelsterbach. I used to know someone who lived in Schwanheim which was 5mins away from the airport by bus. That is close to Kelsterbach. However, the bus is not that frequent.
Rather than going to Lidl, if you have 5 hours in FRA, hop on a S-Bahn (metro) from the basement of the airport and you can be in the centre (at Hauptbahnhof or Hauptwache) in a little more than 10 mins. Plenty of options around there for picking up various SIM's. Other than that, there will be plenty of options in Berlin.
Thanks to Inquisitor too about the pronounciation of o2 auf Deutsch!
