Since Sonarr can’t fetch from a remote file server, you need to use software to mount the FTPS server as a local hard drive for Sonarr to be able to manipulate files.
I’m using a free trial of NetDrive2 which seems to work fine, but I had to move Sonarr to a Windows box to be able to run it. It was running on FreeNAS previously, which doesn’t currently support the prerequisites for sshfs, and will not until FreeNAS 10.