I’m not the guy you replied to but personally I use a setup called split-horizon DNS.

1. I have a DNS server running on a raspberry pi which I have set up as the DNS server for all devices in my local network (by setting it in the router).
2. This DNS server has my domain name as an A record pointing to my reverse-proxy (Nginx Proxy Manager), e.g. example.com would resolve to 192.168.0.100.
3. Any subdomain I want to use is set up as a CNAME record in my DNS server referring to the previously configured A record with my domain. (jellyfin.example.com => example.com)
4. Now all requests to the registered domain and subdomain are routed to my reverse-proxy which I configured to forward them to the correct service depending on the given subdomain.

This is a little bit of a simplification. I also use a cloudflare tunnel to allow access to select subdomains and I have 2 reverse-proxies chained together since NPM can resolve services by their container name as long as they are in the same docker network.

Also probably important: My DNS server was a pi-hole (until today at least) and did not act as my DHCP server. This meant it had no idea of local device hostnames and therefore was configured to forward queries to local device names to my routers built-in DNS server.