I got two U6 and both have the same problem - when mining, the frequency does not rise above 320 MHz. At the same time, consumption is 1400 watts and a speed of 140Gh. I got two D5 for $ 4000.

I dont understand what could be the problem.

Dont you use it in extremly cold or hot conditions?
What Is the chip temperature? My are around 70°C
Ambient temperature + 5 - +10°C. Chip temperature 41 - 43 °C
Try to give it into 20-25°C cos of for example obelisk SC1 was too cold and it runs for 60% of expected hashrate.
I tried to install one ASIC in the box, but after 30 minutes of operation it turned off due to overheating of the chips. The frequency also remained a maximum of 320 MHz.
Weird, and you have 2 machines and both do the same? I have 3 pcs and all have the same freq 550mhz but 2 of it takes 2200W and 1 of it takes just 1900W - with same hashrate and all the other results...
So sorry for that, it was just my hope its the same as the Obelisk.
Yes, both are the same. I tried many different options. Its bad that there is no way to view logs in the firmware.