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Alternatively, use a network analysis tool, capture network communication on the MQTT port between the device and the broker, analyse the TLS handshake and find the “Client Hello” packet. If the packet contains the server’s hostname, it indicates that SNI is supported:
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If the device is not in the local WiFi you can mock a TLS server locally and expose it globally: How do I test locally if my IoT device has TLS-SNI?
How to check if the hostname, port, and access credentials are correct?
Use a different MQTT client, we suggest MQTT-CLI, and try connecting it using the hostname, port and access credentials.