@johanv, check wiki (https://www.domoticz.com/wiki/NefitEasy).
Start easy-server with using --host=127.0.0.1.
No need to specify the port number if you use 3000.
Make sure the PI can access the internet.

I would check the rflink first with the RF link loader. (On a windows pc).
If you see data incoming when you press a switch then you know your rflink is working.
If that is ok you can use syslog on your PI to check what port the rflink is using.
Henk