From the op.
The cable has been tested via a laptop and so assume it’s working ok.
The hubs work when next to each other and a short cable is used.
So I wonder could they be communicating via wifi and not the Ethernet cable when in proximity?
I would switch off wifi on both hubs connect via Ethernet cable as before and then connect the laptop to the second hub, does it still connect to the internet.
If not then back to the guide on how to set the hub up as a repeater.
If it does then your good and should repeat the test but on the long cable run.
If it works on the long cable run then turn on wifi at the far end and see if you can get this to work, then wifi at the source end.