Unable to use TinyGSM to access a long URL

Question

I'm running code on ESP32 (with the A7670 modem) that uses the TinyGSM library. Part of that code attempts to POST to a URL:

TinyGsm modem(SerialAT);
//  ...
//  Code to initialize the modem
//  ...
String server_url = "https://xxxxxxxxxx.execute-api.eu-west-1.amazonaws.com/dev/path/element";
modem.https_begin();
if (!modem.https_set_url(server_url)) {
    Serial.println("Failed to set the URL");
    return;
}
String post_body = "";

int httpCode = modem.https_post(post_body);
if (httpCode != 200) {
    Serial.print("HTTP post failed ! error code = "); 
    Serial.println(httpCode); 
    return;
}

The attempt fails with an error code 715.

Two observations:

• When running the code but using WiFi instead of the GSM modem, all works well
• When running the code still using the GSM modem but with a shorter URL i.e httpbin.org, all works well This leads me to wonder whether the issue is about the length of the URL and the Modem.

I used a URL shortener to turn the original URL into a 20 characters URL, and the code (using the GSM) seems to work (i.e. it returns an HTTP 307 instead of 715. But, of course, I can't follow the redirection since it brings me back to the initial problem)

Could anybody shed some light on this?