I'm pretty new to selfhosting and homelabs, and I would appreciate a simple-worded explanation here. Details are always welcome!

So, I have a home network with a dynamic external IP address. I already have my Synology NAS exposed to the Internet with DDNS - this was done using the interface, so didn't require much technical knowledge.

Now, I would like to add another server (currently testing with Raspberry Pi) in the same LAN that would also be externally reachable, either through a subdomain (preferable), or through specific ports. How do I go about it?

P.S. Apparently, what I've tried on the router does work, it's just that my NAS was sitting in the DMZ. Now it works!