I had no problems finding the summer timetable for the bus
IF you make the 1015 bus from FRA (your chance is 50:50 at best), and IF there are no delays along the route, then you can get to HHN for 1200 which should just about be early enough... but there are too many "ifs" here, and I wouldn't risk it.
Even without changing airports I would never attempt a "connection" on separate tickets in less than four hours... and if the itinerary involves a change of airports I would double that.
Surely there must be better ways of getting from where you are to where you're going?Edited: 08 March 2017, 23:12
Here is the train site in English, if you don't have checked in luggage you might make it, the website shows a connection every 2 hours and a journey time of 105 mins