External Web Server can't communicate with EC2 flask API

I'm really out of my element here when it comes to networking.

I have an EC2 server running linux, which has an active flask API running.

I have a PHP function on a shared hostgator server:

function sendAPI($threadid, $teamid) {
    $url = "http://ec2-...amazonaws.com:9999/api2?id1=" . $teamid . "&thread=" . $threadid;

    $ch = curl_init();
    $optArray = array(
        CURLOPT_URL => $url,
        CURLOPT_RETURNTRANSFER => true
    );
    curl_setopt_array($ch, $optArray);
    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}

Similar function works without issue with the telegram API server:

function sendMessage($chatID, $messaggio, $token) {
    $url = "https://api.telegram.org/bot" . $token . "/sendMessage?chat_id=" . $chatID;
    $url = $url . "&text=" . urlencode($messaggio);
    $ch = curl_init();
    $optArray = array(
        CURLOPT_URL => $url,
        CURLOPT_RETURNTRANSFER => true
    );
    curl_setopt_array($ch, $optArray);
    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}

When navigating to http://ec2-...amazonaws.com:9999/api2?id1=1&thread=1 from my own web browser, the API picks up the request successfully.

Inbound rules on EC2 instance:

Note, website IP, also the same IP is listed in CPanel:

Domain Name: novociv.org Top Level Domain: ORG (Organization) DNS Lookup IP Address: 108.167.142.88

From my limited knowledge of networking, this should all work correctly. However, when the PHP function is called it returns a null result and the EC2 server does not react. I don't know how to troubleshoot from here because I don't know where to find any error messages. I would guess that the EC2 is simply blocking traffic from the external server, but I don't know what's wrong (if anything) with my inbound rules since the EC2 successfully picks up requests from my laptop.