Hello @yassk ,
Based on the information provided, it looks as though we may be attempting an SSL connection, and receiving an ‘Operation not allowed’ error in response. In the code snippet above, it looks like we are using the TinyGSMClient. Reviewing some documentation and other users inquiries both here on the Community Forum and on Arduino forums, it looks like TinyGSMClient may not offer SSL/TLS support natively. This would prevent you from supplying the server or client certificates on the MQTT connection attempt, preventing a SSL/TLS connection.
That said, it does appear that there are some libraries that extend the features of TinyGSMClient, such as this public repository. Please keep in mind that this is not a library supported or maintained by HiveMQ, and as such, implementation details and configuration will best be handled through the repository or by repository contributors.
Once the MQTT connection request for this device is able to utilize TLS/SSL and provide certificates, the server certificate for HiveMQ Cloud Brokers can always be obtained on our Frequently Asked Questions page, or linked directly here.
Best,
Aaron from the HiveMQ Team