I started dex 6 weeks before FET. Cycle day was not relevant. I took dex for 3wks then switched to prednisolone.
Your RE's reaction is no surprise. I highly recommend you contact Dr Matthias. He does phone consults for non-Syd based patients and sends all the BT forms and prescriptions by mail. The immune protocols he uses are individually tailored, there isn't a blanket approach that will work for everyone.
Best of luck to you!