please make sure those cables are not crossover cables they look the same as regular ethernet but should be marked accordingly. 

Okay. I will check. I’ve never heard of such. So I appreciate that information very much. 

Confirm those existing Ethernet ports have rj45 connectors and are wired for Ethernet as opposed to wired for phones. I believe wired for phones would have only 2 wires to the connector. 

So I haven’t been able to find anything labeled as “crossover”. But here is the title for one of the cables I tried: “

RJ45 Cat-6 Ethernet Patch Internet Cable”

Plug them in directly to the gateway to confirm they connect. If they connect directly to the gateway, your in wall wiring is the problem. 

This is a very good idea browndk26. Thank you. I'll post an update when I've had the opportunity to test the cables. 

Easiest way (for those with decent eyesight or a camera on their phone!)

take a picture of both ends of the cable. (this won't work for wires in the wall!)

The answer for that is a Ethernet cable tester (prices from $7.50 to $7,500.00 on amazon!)

The more you spend, the better the capabilities of the unit!

turn end of cable so the prong is face up.

Look at the wire colors  from left to right.

if the colors are all in the same order it's a 'normal' cable.

If the colors reverse at some point, it's a crossover cable.

Now comes the fun part.  You may STILL need to get an ethernet tester.

take your device to the closet where the fiber router lives.  Plug it in DIRECTLY to the Ethernet port to see if its a router or wiring issue.

UNFORTUNATELY (as someone who spent almost 20 years doing on-site low voltage wiring installation and  troubleshooting)

A good deal of the apartments/townhouses  I have seen recently have had poor or broken Ethernet installations. (ranging from mis-wired jacks, to broken wires mid-run).

I'd say if you can get a valid connection at your fiber modem, you then need to complain to your builder or apartment manager to get

someone to test your wiring.

SIGH!

I finally got the chance to check my cable by plugging it directly into the router. It worked. It is a bit hard to tell the colors in the end of the cable, but I can tell that they are all different colors with red being at one end and orange being at the other. So no pattern either way. My guess is that I’m going to have to call the builder on this one. Thank you for your help. 

I have a similar issue where my wifi is working without issue, but the three ethernet connections have issues every several days that require a modem reboot. I ran 50 foot cables from the modem to my workstation and to a bedroom Apple TV. I have a third cable that is 10 feet that runs from a second Apple TV to the modem. This has been an ongoing issue since we switched to 1Gig back in November. They have replace the modem once to see if that's the issue. I have not changed any of the default settings on the modem.

Any help would be appreciated.