Calculate resonance mode of a cavity which operate at 169-171 GHz wicth CST

Hi
I want calculate resonance mode of a cavity which operate at 169-171 GHz wicth CST

I use from Eigenmode analysis but number of mesh is much.I reduce the mesh at min but progress of my simulation after 2 day be constant.

Please offer a way to solve this problem and if there is another way for calculate the resonace frequency, please propose.