How do I add different brand onvif camera's to my Flux device?

To add an onvif camera to your flux device navigate to the Main menu-> Select the option "Add Camera" (found under the Camera section) -> Select the option "Add Manually" -> Change the protocol to "ONVIF" and fill in your cameras corresponding information (including user, password & port).

You may click here for a good reference guide with an example when attempting to add a UNV IP Camera.