If the engineer checked the line and told me I'd get the max download and upload surely that means the connection to the cabinet is fine. I've tried factory reseting 20+ times and nothings changed. I don't think 6.5 is a close estimate to 13.5 at all. That's worse than half, 'estimate' or not.
The shorter cable would not make a difference unless it is faulty. You could always swap them back just to be sure.
@john46 wired, with two pc's. Test was also tried on a Samsung S3 on wireless, same problem.