To do that please follow the instructions:
Connect COM port only locally on single device using simplex fiber cable - this will create internal loopback for all video channels and channel 1 will appear on output 5, channel 2 on output 6 and so on.
Please do the same for each device.
This will allow us to see which device has a problem and if it's a problem with transmitter or receiver.
Only loopback the Com 1 port as shown in the panel diagram below. 
