My guess for the ping is, that the domain resolves to the IP of your OPNsense-router, hence it’s not your server responding, but the router.

I haven’t set up port forwarding for OPNsense, but my guess would be it’s only working for connections from the wan interface or some local rebind protection. You could set up a DNS-override in the unbound settings for your domain and redirect to the internal IP-address, no need to set up an additional DNS-server.