@ldecicco, last autumn I started working on a somewhat similar task using the same radio module, but for posting to Azure EventHub.
As @srgdamiano mentioned, the key is to add HTTPS capabilities using the
TinyGsmClientSecure client(modem) . Doing that worked for me and I was able to Post data.
Although I haven’t worked on this for a few months, I need to get back to it and I just created this Pull Request from my feature branch to the develop branch, so that you can compare my code changes: https://github.com/EnviroDIY/ModularSensors/pull/432