In firmwares before 3.1-1605 it was in Control Panel > Web Services > HTTP Service Options > Configure HTTPS then select "Enable HTTPS connection".
With 3.1-1605 you have to go to Control Panel > DSM Settings > HTTP Service then select "Enable HTTPS connection".
At least the automatic redirection from HTTP to HTTPS works now!
Along the way I was getting an error
- SSL received a record that exceeded the maximum permissible length.
(Error code: ssl_error_rx_record_too_long)
This turned out to be because I was trying HTTPS on the HTTP port 5000. As soon as I tried HTTP on 5000 connection was OK.
Also, the firmware simply will not upload / update over HTTPS, so you need to turn HTTPS redirection off (now it works) to be able to update the firmware.