Thank you. That was very helpful.
But this leads to my second problem. I have a feed and reflector antenna system. And because the model is large I am putting feed and reflector in different FE-BI regions. In earlier versions of HFSS I was able to use Direct Solver (which I always prefer, based on my experience) to run this kind of model. But in v16+ the direct solver option is inactive and it is forcing me to use Domain Decomposition Method (which, in some cases, is not converging even after 200 iterations).
Is there a way to run Direct Solver with FE-BI regions in HFSS?