Aha! I did not add the "D" to the model number, and my Google-fu utterly failed.
Apologies
Will now go watch a cat video on Youtube as penance.
Also, on my Freesync monitor I had to go in to the actual monitor menu, using the controls on the monitor.
Enable Freesync there. NOT in Windows/Radeon settings.
Reboot. The option in Radeon settings then became available.
I believe this is what the OP needs to do.
Hope that works.