I am sure this was replied. unfortunately the system did not catch it up, which can happen sometimes. sorry about that.
FDE definitely only simualtes a cross section of a waveguide. it is not 3D solver. In your case if the horizontal geometry is a cylinder, not a rectangle, you will need to use 3D FDTD to simulate such cavity. In case you only need to simulate 2D cross section to get some idea, please consider using the same dimension as the 3rd party, and similar boundary conditions. What you got currently is more like a metal box mode. In addition it looks like such mode is similar to the whisper garllery mode, which has many different modes with small wavelength spacing and very close neff. therefore you will need to carefully set the simulation region, mesh and boundary conditions, as well as looking for higher order modes to identify the true propagation or cavity mode.
You can use either PML or metal BCs. However to know which one is the correct mode using metal BCs, you will need to change the simulation dimensions and see if the mode profile and neff chage. For PML, you may need to have more number of trial modes to get out off PML modes.
The frequency you use is not the same as in the first post you use 193.11THz and in FDE you use 193.414THz. Please note that Lumerical does not use the norminal light speed, it uses the real speed in vacuum:299792458 m/s based on permittivity and permeability. Therefore conversion of wavelength to frequency will be different from the norminal speed. Bear in mind that the acutal quantity used in the solver is frequency, not the wavelength.
"selective mesh" is not Lumerical's concept.
Please modify the settings and try different simulation dimension and boundary conditions and make sure the solver is correct for such simulation: cavity or waveguide?