If the phone switches networks, then it would have to create a new connection. The router can behave the way you describe by having a server listen on port 80 (HTTP) and then proxying data to a new session on HTTPS.

There is the SSLStrip attack that is similar to your description but it involves many more steps.

If the phone switches networks, then it would have to create a new connection. The router can behave the way you describe only by having a server listen on port 80 (HTTP) and then proxying data back and forward to a different session over HTTPS.

A simple configuration issue cannot produce the necessary servers and proxying behavior for your scenario to be valid.

There is the SSLStrip attack that is similar to your description but it involves many more steps.