If "hi" object doesn’t work look at the serial object, that should do it. It’s a more general-purpose serial communication object (as I understand it), the hi one is for certain types of interface hardware that the OS recognizes like keyboards or joysticks, etc. Don’t know how your specific hardware behaves. You may need to switch between ports to send messages, or perhaps multiple serial objects set to static ports would do it. I’ve only used one serial at a time, for an Arduino board. Works well.
Sounds like you’re closer but still not getting what you want. If you could send a link to the fan model and documentation maybe that would help. Also you may need to send your commands to the fans as ASCII (? not sure–) try sending the numbers (like 251 or 255) through the "spell" object first. I’m not sure if this matters though.
Also use the "print" command to the serial object to see the port list in the Max window. You should be able to tell how many ports there are in use when the fans are connected. Whether the fan device uses 1 or 4 ports, I don’t know, but I would think it’s just one and different commands can be used, hence your number commands. Also, are there other commands than just on / off? like speed commands?