Router to router connection failing, long cable

Question

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.

Answer 1

So the problem seems to be that routers nowadays expect cable length to WAN ports to be small.

The solution for me was to use a good enough switch (TP-Link's TL-SG105 in my case) to sit between Router 1 and Router 2, so the setup is now

• Modem ← 1 m cable → Router 1 WAN port
• Router 1 LAN port ← 90 m cable → Switch port 1
• Switch port 2 ← 1 m cable → Router 2 WAN port