I am not an expert on security and I don’t know how to setup ddns, but I simply use Tailscale VPN to access my RPi4 homeserver running Umbrel from outside of my home, I simply installed Tailscale in my Umbrel and in my phone, and when I want to access from my phone Immich, Nextcloud or any other app running on my Umbrel. I simply enable the Tailscale VPN. Maybe it has some limitations, but is the simpler way I found, and based on what I read, it is more secure than exposing a port in the router, but not sure.