If you paid anywhere near what is shown on that ebay page then you are a brave man.
I paid ten times that for my Acewell, but it works

I hope it came with better instruction that are shown on the ebay page.
From what I can see Tacho sense wire is Violet/Black, Speedo sense wire is White/Black
All you should need to do is connect this to the Grey wire that connects to the original dash, or to the Yellow/Black wire on the main loom connector, then start the engine.
I assume that other functions of this dash work and that the original tacho was working?
Do you have a multimeter to test some voltages?
Fault finding...
Connect power to the new dash unit.
Connect a wire to the RPM terminal and then tap it repeatedly on to the 12v feed to create some pulse input to the unit.
3 taps per second is equal to 180 RPM, so you should be able to get something on the display.