I have had my Archer AX21 wifi router for a year and it has worked OK. Recently, it works OK with wifi through my phone, using "speedtest" I see over 100 Mbps which is acceptable, but my wired gigabit ethernet has become drastically slow. Previously I would see between 150 and 350 Mbps download speeds over ethernet but I am now seeing speeds reported like 0.18 Mbps (!) which is obviously not adequate for smooth videoconferencing, which I need to do.

What could be causing this, and how could I diagnose it?  I just updated the router firmware with no change. I've rebooted various ethernet switches, with no change. I have examined the router's admin screen "Clients" page and I do not see any clients with data transfer "Real-time Rate" as particularly high, in fact they are only kb/sec rates. I have a number of ethernet clients, I think about 15 on the network, but as mentioned they had been working before, and I did not change network topology or add new devices. My cable modem can't be the cause, because the wifi link has OK throughput to the internet. Does this behavior ring a bell with anyone?