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Here is the scenario:
- Router 1 is plugged in to a modem in its "wide area network" (WAN) port.
- I connect one of Router 1's "local area network" (LAN) ports to Router 2's WAN port via a very long CAT 6 cable (around 90 meters).
- Router 2 fails to detect the connection; it insists that there is no cable connected to its WAN port.
I tried many things, including different router brands (Netgear, TP-Link, ...), and eventually I discovered that if I do the same Router 1 LAN port to Router 2 WAN port connection using a 1 meter cable, everything works as expected.
What to do?
I am asking and answering this question as it could have saved me a few hours of my life if someone had asked it before.
1It can be: wrong router (instance, not model), wrong/damaged cable (twisted, crushed, short, open, etc., during laying, for example), wrong/damaged jack/socket. – Akina – 2019-04-19T12:12:01.237