I do want to add that defective Ethernet cables are more common than one might think. I wasted about a day troubleshooting a problem before finding a defective generic Cat 5e cable. When I replaced the cable with another Cat 5e cable, the problem was solved.
After that, I bought an inexpensive network cable tester from Amazon. It will not test data rates but at least does a thorough job of checking continuity and correct wiring.