Originally Posted by mact
As I said in the OP, the IP address is unchanged, otherwise SMB connection and VNC would not work. And they do.
You really allow SMB connections from the internet?
You said you had to open a port, so connections would be coming into your net from outside. The IP address that is probably changing is the one for the internet-facing side of your router. Certainly using calibre's test wouldn't detect that, and neither would VNC if run from inside your net.
I have a calibre server that has run for weeks at a time with no problems. This argues that there is something in your network configuration that is causing the problem. Alternatively, I am lucky, which is possible.