I am a bit of a NOOB with Pi's, though I've gently used them since day 1.
A Pi 3B+ purchased maybe a month ago from an official supplier, with a 8GB SD card, absolutely standard (and up to date) Raspian OS, with a standard keyboard and mouse plugged in to 2 USB ports. Using the official white Pi3 PSU.
My current issue / observations:
1) When connected with a short ethernet cable to a Netgear GS108T switch the ethernet socket led lights come on and the Desktop network icon shows 2-way green arrows. Ok Good.
2) When connected by approx 10-15 meters of "reasonable quality" cat5e cable from the Netgear GS108T switch the ethernet socket led lights come on and the Desktop network icon shows 2-way green arrows. Ok Good.
3) Increasing the cable distance, to approx 25 meters, then the ethernet socket led lights do NOT come on and the Desktop network icon shows red crosses. FAIL.
The max distance for ethernet cable, i believe to be 100m, so well within this.
I think that the yellow/green led's do very very faintly glow during boot, but never come alive.
I have tried exactly the same 30 meter cable using a standard PC -> all Ok and most significantly using my older Pi 2B and Pi 1. Both work perfectly.
So, am I missing something silly, or is there an issue with the 3B+ (ie possible hardware issues, or a sensitive adaptor, etc)?
I hope you can help,
PS: Sorry if I've posted this in the wrong section.