FS#3986 - Autonegotiation stopped working

[openwrt-bot]

Technocrat:

Supply the following if possible:

• Device problem occurs on ASUS RT-AC65P
• Software versions of OpenWrt 21.02.0-rc3
• Plug WAN port with two-pair cable of an ISP (100Mb/s)
  Execute cmd:
  ethtool wan

Speed: 1000Mb/s
Duplex: Full
The connection is not working due to the incorrect speed.

Connect the same cable to a switch, and then the router to the same switch. Or execute the command:
ethtool -s wan speed 100 autoneg off duplex full
After that the connection is properly established with proper speed:
ethtool wan

Speed: 100Mb/s
Duplex: Full

Autonegotiation was working properly about a year and a half ago, so there was some changes made in the code that affected it. It also working with no issue with stock firmware.