I have a WRT1900ACS and a WAC104 on either end of the house that are connected together with CAT 5e. With 21.02 I replaced the Netgear firmware with OpenWrt on the WAC104 (both devices use DSA). The WRT1900ACS has been running OpenWrt for a long time.
After running for days, I was losing the connection between the devices. I would power cycle both to fix. On the third occurrence I looked a the logs.
After running for days, the WRT1900ACS would spontaneously start negotiating the link speed on the port to 100 FD and the WAC104 would negotiate 1000 FD, over and over.
I fixed this by installing ethtool, and giving the command: "ethtool -s lan2 speed 1000 duplex full autoneg on" I also added that command to the startup script /etc/rc.local on the WRT1900ACS.
Is this the correct way to set speed on a port with 21.02?
P.S. I never saw this problem when the WAC104 was running the Netgear firmware.