Except 1043ND always "Unable to complete PPPoE Discovery

Hi All,

there is some weird behaviour of my routers,
it is probably normal? And I just dont know some technical reasons/specs?

Short description:
Ethernet WAN cable from ISP - only 4 veins, not 8 as for gigabite: WhiteOrange (core 1 of 8), Orange (core 2 of 8), WhiteBrown (core 3 of 8), Brown (core 6 of 8).
My main router is TP Link 1043ND and OpenWrt v17.
While ISP cable is connected into WAN Port of TP Link 1043ND, the router shows the port in Luci as "not connected/no link",
despite of it, PPPoE discovers the ISP correctly and I have Internet in my house (WAN blinking normally).

At the same time, all my other routers - EasyBox 803 (openwrt), Linksys WRT54G (ddwrt) and Zyxel Keenetic OMNI (factory firmware) cannot discover PPPoE through their WAN-Ports.
So all these routers just shows in their System logs:
pppd[]: Timeout waiting for PADO packets
pppd[]: Unable to complete PPPoE Discovery

What is the issue? Is 1043ND-WAN port has some special possibility? I am pretty sure the ISP-cable shouldn't be DSL or smth like that.

Thank you.

This topic is related to How to turn first-LAN port into WAN? - Installing and Using OpenWrt / Network and Wireless Configuration - OpenWrt Forum