I needed to connect two buildings and was having machines in to dig a 4' (1.2m) deep trench between them for a water line so I went to Amazon and bought a 250' (76m) pre-terminated copper Cat6 cable. As I was going to be burying it I wanted to be sure it worked, so I used it as a "fly lead" for my laptop for a week or two first and it worked fine. I know it initially connected at 1Gbps, but (stupidly) I can't be 100% certain it stayed at full speed the whole time.
Now that it's buried I'm only getting 100Mbit/s. It does sometimes connect at 1Gbit/s, but it later falls back to 100Mbit/s. I have an old Cisco SG300-10P on one end and a Ubiquiti Edge Router X on the other. I disabled 802.3 Energy Efficient Ethernet (EEE) on the Cisco and, as expected, it made no difference. The Cisco has built in cable test capability and it says I have an 84m open cable on all pairs - even when connected to the ER/X and working. Is there some sort of loopback/test termination I can make for the other end to get a better (more meaningful) result? I've tried searching, but failed.
The plug at one end did get pushed through some silicone caulk as it was being shoved through a hole in a wall. I cleaned it off with alcohol and it looks clean, but I'm considering cutting the plug off and replacing it with a socket as my next debugging step as it would be more convenient anyway.
I live about an hour from the nearest large town so there's no way I'm getting someone here with a proper tester at a reasonable price. If I can't figure it out myself I'll revert to the pair of airMax GigaBeam radios that have given me a solid 800Mbit/s for the last 3 years with only visual alignment!
Edit: this is the cable https://a.co/d/i6mYLy1