When I flew in/out of FRA in 1988, I had the impression that the Regionalbahnhof was under the terminal building (it might have been, then). Since then I have come to realize that it is under the building across the access road from Terminal 1. The best way to get to it is to go to the great hall of Terminal 1 and take the stairs/escalators from just inside the outside wall of the terminal to the mall under the street which leads to the Regionalbahnhof.
The Fernbahnhof is further out from Terminal 1. There is an enclosed bridge from the building across the road, over an autobahn, to the Fernbahnhof. In the past I've gotten to it by going up to the second floor mezzanine in Terminal 1 and across the pedestrian bridge over the access road, then through the building and out the enclosed walkway. Last fall, my route from the new A/Z concourse took me into the Regionalbahnhof, and I followed the signs from there to the Fernbahnhof.