Many thanks for your help, Daniele!
Unfortunately, port scanners still indicate that the ssh ports 22 and 2223 on my router are closed. I also tried briefly setting the firewall to level to ‘low’, but this seemed also not to help (unless I need to restart the router each time?).
The port forwarding seems to work fine from a computer connected to the LAN, but as mentioned, the ports seem to be closed to external devices according to port scanners.