I’ll jump right into it, as I have a similar use case (garden irrigation system - about 600m away from my flat with 4G LTE Router).
I am using an esp8266 as hardware and everything works fine on local IP adresses (at home) and I also have a dynamic DNS service.
With Port 8080 everything works, but as soon as I try to use SSL (Port 9443) the ESP8266 tells me in the Serial Monitor that it tries to connect the time (but never ‘connected’).
Important to know: at the moment I have nothing special set up on the server regarding SSL.
My home server runs nextcloud and letsencypt already.
Yesterday I tied to create a self signed certificate but if couldnt be loaded and kept saying:
Error initializin ssl context. Reason : File dies not contain valid private key: …"
(already tried everything mentioned in this thread
Any hints and help is highly appreciated