It’s possible that, when the ISP revokes the public address and assigns a new one, the DNS record isn’t updated immediately and still points to the old address. Then every new request would be sent to the old, invalid address.

And this is where I start shilling for Tailscale. It’s a Wireguard-based mesh VPN that is designed to work from behind firewalls, NAT, and CGNAT. It has its own internal split DNS provider, and probably some mechanism to handle public address changes that is transparent to the tunnelled traffic. You can use it to share the server with only the devices that have the client installed, or expose the server to the internet.