Hello!
With Dyndns you can just use their web interface and update your IP whenever you need to. I use their update client on my iMac and it does it for me. It doesn't matter what machine it runs on inside your lan.
To test your BBS internally just telnet in to the LAN IP instead of the Dyndns IP from another system. This needs to work (obviously) before messing with the NAT.
Yes the port forwarding is done on your router, just forward telnet to the IP of your Amiga and you should be good.