The tunnel is the interesting part

Self hosting usually breaks on the network, not the server.

Home connections sit behind carrier grade NAT, addresses move, and forwarding a port means exposing a machine on your own network to the open internet. An outbound tunnel sidesteps all of it. The phone opens the connection, so there is nothing to forward and nothing to expose.

The side effect is that the phone works exactly the same on mobile data as it does on home wifi. Move it, and the site moves with it.