Yes, you will have to go landside to register. The point is, you go through passport control after registration rather than before. You may have to go through security regardless, though that can be unpredictable depending on the specific airlines and destinations and gates involved. The classic
advice on connecting at FRA applies as always:
Just follow the signs. Don't think. Don't apply logic. Just follow the sign. You will get to your destination. You might have to go through security 2-3 times, customs, passport control, tunnels, escalators, elevators, stairs and you feel that you have just walked to your final destination.
But under no circumstances try to make sense of it. You will only get lost.