How do I calculate limit frequency of the attenuator circuit?
Assuming that the questioner really wants to ignore Rg's effect on the frequency response and starts from Ue:

Replace Ue, R and the diode with their Thevenin equivalent:



The most band limited case is with the highest attenuation, when rd is the smallest, practically =0 Ohm for a small signal AC.

In the high attenuation case the RC highpass filter RL &amp; C2 is the decisive part. Solve the fg-candidate from RL and C2.

If you know something spesific about the diode, you can calculate the rd, In that case you can consider this to be a RC highpass filter that has the resistance = RL + Parallel(R,rd)