Finally I understand your questions.
As a FDTD beginer, please watch some videos:
Then try to build the ideal WBG device: just use 5 to 10 periods, otherwise it takes you too long time. Unfortunately Ansys does not have a complete example with a few periods to refer. Usually the device is simulated by EME: https://optics.ansys.com/hc/en-us/articles/360042304234-Phase-shifted-Bragg-grating
The basic 4 steps in setting up a simulation file are:
1: set up the geometry and select the material. in ideal WBG they are rectagles with different sizes. for lithor siulated, you have the unit cell, and just need to import it, then copy and move it periodically. just for 5 periods to be efficient. remember to have a substrate.
2: add a mode source or port, modify the source spectral bandwidth
3: add FDTD, modify its size to include the WBG, leave about half wavelength between PML boundaries and the device.
4: add power monitors if you use mode source, or simply add one time monitor if you use port.
then save the file, check the material fitting memory requirement, and run. After simulation you can try to visualize reflection and transmission.
For actual WBG it has too many periods, FDTD may take longer simulation time (and you need to modify the simulation time settings from the default 1000 fs to more) and more memory. This is why I suggest to check the memory before running a new device simulation.
You may have different questions during this process. Please search the forum and knowledge base to get more information. In case you could not find a solution, please post your questions one at a time here and I will help you.