Hey @F_Gaudon ,
Nice to see more people getting interested with MQTT and HiveMQ, welcome to our community
HiveMQ Cloud cluster doesn’t need client.pem and user_key.pem. HiveMQ Cloud cluster authenticates the client by username and password (so called “simple auth”). In order to “trust” the server, you device might need a root CA cert. You can download the root certificate here. This will create a file called isrgrootx1.pem, which you can use as “Server Certiciate” (cacert.pem)
Your device should also support TLS-extension called SNI: Server name indication.