Had the same situation literally just yesterday
First make sure you can connect to the mixer: Open the control panel (C:\Program Files (x86)\Denon DJ\X1800 Prime\Panel.exe) and see if it shows “Connected”. If it doesn’t, uninstall and reinstall the x1800 driver.
Updating from 1.2 to 1.5 wouldn’t work for me. I eventually had to download the old 1.3 updater, which did work, then update to 1.5 from there. Updating takes a while (like 10 min) but you’ll know when it’s working if the gain bars are moving during the “data receiving” stage.