Need to find out about the setup then!
CPU?
USB 3 Risers?
Motherboard?
Number of GPUs?
Type of GPUs?
Ubuntu Version?
Do you have one GPU acting as the display output or are you using onboard graphics?
PCI-E settings?
6600K
MSI Z170 Pro
2x 1060, 2x 1050 Ti (USB3 Risers)
Ubuntu 17.04
onboard output (tried GPU but doesn't work either)
Gen1, 4G decoding
You need to have identical models for Ubuntu e.g. all GTX 1060 or all GTX 1050 TIs.
In your situation: you will be limited to using Windows 10.
Did you get all 4 cards installed or where some missing in Nividia Xorg Server?
All cards are working without OC.
OK, overclocking and fan control can only be enabled when a monitor is connected to the GPU on primary PCI-Express slot.
Set it to PEG in bios. Make sure all the PCH is set to Gen1, and check if you also have the option to set the individual slot selections to Gen1.
Try starting with a GTX 1060 on primary PCI-Express slot.
If, that does not work, then try starting with GTX 1050 TI in the primary PCI-E slot.
As soon as you get a display output from one GPU connected to a monitor - you will be able to set fan controls and overclocking.
Also, see if Disabling above 4G decode works, it sometimes causes a black screen.