Cable length from router to 2nd base station

I am installing a 2nd base station in our basement. The cable I’ve pulled is something around 300’ in length. I’ve tested the cable and, as far as the test unit is concerned, the cable is good. However, when connecting the 2nd base station to our router, Office Manager shows the unit to be offline. I’ve been told that the cable is not good and should replace it but the cable tests good. All this makes me suspect the base station rather than the cable. Suggestions and comments please.

Well you are right at the end of the spec of 100 meters. Take the 2nd unit back to the router and connect a 6 foot LAN cable, does it work? Now connect a 100 meter cable to see if that works.