Xiaomi 4a router (gigabit edition) Internet not working

fyi, I just reinstalled 21.02.2 onto my R4AG. Got openvpn client working.

I then moved the R4AG to another room, to temporarily replace another model of router, and guess what. No internet !

The only difference is I set up the R4AG when WAN port was wired to a gigabit switch which is the one I normally use for testing. The other room where R4AG reports no WAN connection has a 100mb switch (TPlink SF1008D). The switch and cables are fine. I ran a long cable from the R4AG to another gigabit switch and WAN access was restored.

I had come across another thread where someone was reporting a router would not negotiate a link when wired to a 100mb port of a modem. I don't know if it is for a R4AG at this time. I need to find the thread, unless another reader knows which thread I am referring to.