Content server access remotely

[Tlman46]

I want to be able to download content from my Calibre library while traveling.

So I need to have my router forward the port that the Calibre server uses, to the PC running Calibre. I don't want my desktop exposed to the outside world, so I built a small Raspberry Pi machine and installed Calibre on it, and installed it next to the router.

I use a Hitron cable modem/router. I switched it to 'bridge' mode, so there is no NAT or firewall function - those functions are supplied by my own router. So now when I look at the WAN address in the router, I see the outside world's IP address, not the old Hitron router-supplied address. I am 99.9% sure there is only one stage of NAT now, that in my own router.

In my router, I forwarded the appropriate port to the address it reserves for the R-Pi box.

In my tablet's Calibre Companion setup, I set up a second route to the server, now the external IP address (which with cable is static) and port.

When the tablet is connected to my LAN, it can see and access Calibre content on either machine, my desktop or the R-Pi.

But when it isn't connected to my LAN (at home, if I turn on a hotspot on my phone and force the tablet to connect to that wifi SSID), it cannot connect to the content server. It says it can't see a server and suggests I should confirm the server function is turned on.

I'm flummoxed! The fact that I can access the R-Pi from my LAN says the machine, program and server functioning are fine and the port assignment is correct. I'm sure the router is seeing the outside internet without an intervening firewall. I'm sure the port forwarding on the router is set up correctly.

What might I be missing?