Assuming the Server is accessible IN HOUSE'
Yo need to 'Port Forward' the Calibre server port (the default is 8080 BUT I would really change that to somethin else in the 808x range to begin with.)
Port forwarding requires 2 things (for the setting). 1) the IP of your system (shows if you right click on the Globe with 3 balls icon) and the Port.
That gets set into your router (contact you Router maker, RTFM, or ISP (if it is theirs) on how to do that step. It differs all over the place.
Once that is done, all you need to know is the ROUTERS (outside) IP. (
whatsmyIP.com)

I only have a IPV6 according to that.
speedof.me does report the IPv4
Then when you are on another network (not using your router) <<<<<<
the address will be that
IPv4 IP:808x x being the Calibre port setting you used

Set up a Calibre user Password in Sharing over the net.
Note:
Your computer can not sleep (or it can't answer)