I originally tried using a cat7 30meter length.
I have since tried a brand new 20 meter cat5a.
But the ports are working with shorter 5m / 10m cables.
Any idea why this is?
I think you have just got unlucky, cat5e will run internet for up to 96meters. Are these homemade or bought patchleads? I can think of no logical reason why a router port would only work over a short distance.