Port forwarding to transmission

[Fred]

You have to modify the settings.json when transmission is not running. I believe transmission overwrites this file every time you shutdown transmission. Don't know if you were already aware of this.

So:
Shutdown transmission.
Edit settings.json.
Start transmission and try again.

If it still doesn't work try disabling the rpc whitelist in settings.json and try again, just to make sure the problem lays here.