When results vary with the mesh, we recommend to perform convergence testing. This means that your mesh should be fine enough such that results converge. Please note that sometime the problem might not be due to the mesh accuracy, but due to PML poor absorption (which its performance will be affected by the mesh size).
To accurately mesh your structure, you need to have a minimum of ~5 mesh cells for every geometry. You also need to check and make sure that mesh cells overlap with the geometry by looking at the edge of meshed structures. This is a good link for your reference:
Also, you can try different mesh settings for the structure. I recommend to start with conformal variant 0 and then conformal variant 1 (since you have metal objects here). Here is a good link for how to choose a mesh setting:
I will be happy to take a look at your meshed structure and results if you could upload your simulation file.
I hope this was helpful.