Hello @YC-Michael ,
Thank you for the outreach!
While at this time we do not have known restrictions for M2MQTT Unity, there are some requirements when attempting a connection to a HiveMQ Cloud broker. Namely, the connecting client must support TLS, and TLS-SNI.
Additionally, some connecting clients require the provisioning of a server certificate file. This can be obtained directly through this link, or via our frequently asked questions page here.
If this does not resolve the issue you are encountering during connection, please feel free to provide the specific error you are receiving when attempting connection, or other supporting logs that may help us identify the reason for the connection failure.
Best,
Aaron from the HiveMQ Team