You need some type of converter like this https://vagtronics.co.uk/product/tacho-rpm-converter/ Do you have standard ECU? Use search, there are more posts with this problem. https://www.clubgti.com/forums/index.php?threads/rpm-converter-mk2-golf-1-8t.273171/page-2
it shows completely dead or shows a very low RPM? Mine was showing nothing at all and turned out i had a bad earth to the clocks. Is your tacho converter definitely wired correctly?
It shows about 200 when idle, when i rev the engine it goes ut to 2-3-4000. So it works. its only little low when idle.
if its a GTI originally with MFA cluster there will be a male spade near fusebox with brown and brown/white wires in it. make sure this is earthed, originally it would be in engine loom off to the head, but can earth to chassis or claw above fusebox if you like.
also if you plug vagcom in you can check engine speed measuring blocks, compare this to cluster rpm. see if it really is only off at idle or if its 800 or so low across the range
