HTTP post via GPRS (SIM808) to Azure http endpoint logic app is failing
Juraj's comments put me on the right track, for SSL you need to do more then just select the port.

I did some searching and got it to work using a different GSM library that also supports SSL:
tinyGSM

in combination with ArduinoHttpClient

Jeroen

EDIT: It works, but there is a big downside. It looks like there is major overhead, each message seems to uses 13Kb of data...? I suspect this comes from the data that is returned by the endpoint.
This is way too much over GPRS so now looking into MQTT.