@MontyJ
Some remarks (R) about your post:
(R1) calibre-web and PPi4 is a nice combo. Run it from an USB-SSD not from a SD card.
(R2) No-IP free or paid? Take a look at duckdns.org or freemyip.com for
FREE.
(R3) SSL = security = great! Why not use Let's encrypt?
https://pimylifeup.com/raspberry-pi-ssl-lets-encrypt/ Better than self signed certs!
(R4) https:// = port 443 (in your setup you have forwarded port 80 to 8083) This does not work. How you get your modem/router login screen is beyond my understanding..... unless you modem router is open to the www.
NOT GOOD!
You should forward port 443 to port 8083. No guarantee it will work.
The portforward 80 to 8083 should be closed.
Please report back if this works. If not we'll go the Letsencrypt route with reverse proxying in apache2. (Easy if you know how, but the learning curve is not flat.)