ETHERNET connected but NOT working in Big Sur (Mac mini, M1 2020)

Two important things to keep in mind:

1) No two computers in the same local network can have the same IP, so admitting your PC was 10.0.0.1, your mac CANNOT have the same address.

2) When you have a 169 address it means that your computer is not being able to connect to a DHCP server. It is not surprising that it was not working earlier, and that DNS field is empty, because that would have been filled by the DHCP server.

So, for some reason, and regardless of you choosing DHCP or DHCP with manual IP address, your mac and router are not communicating properly.

You could try temporarily connecting to a different port on the router and see if that changes; even better, switch ports with that pc that is working, and see what happens.

Another thing you can try is using a different cable, but given what I see above I don't think the cable is the problem in this case.

What are the settings for Ethernet? DHCP?

1) Select Ethernet in the left side and post a screenshot.

2) Click the Advanced button, then click DNS.

What do you see there?

Only some Ip addresses in grey? Or in black? Post screenshots.

If there something in black, press the - button.

If it is in grey, click + and type 8.8.8.8

Try to reset SMC & PRAM (it cured Ethernet on my macMini 2018 a few months ago when my next step would have been to fiddle with the Network settings. I had already re-connected the cables and reset the router).

It seems it is no longer possible to reset SMC or PRAM on M1 Macs.

https://eclecticlight.co/2020/11/28/startup-modes-for-m1-macs/

NVRAM is available in the M1 Apple Silicon Chip, but the machine is configured to automatically run test on startup and automatically reset the NVRAM if needed.

https://www.iphonetricks.org/how-to-reset-m1-mac-smc-nvram-pram/

First, is there some reason you need a manually configured IP address.

Two, usually, the IP address whose last digit is a 1 is reserved for a router/DHCP server.

Is 10.0.0.1 really the address you want?

I totally concur with woodmeister 50.

Your settings are unusual.

My guess that all you need to do is change from "DHCP with manual IP address" to "DHCP", and let the mac and router configure the connection properly.

Another clear indication that the settings are wrong is that there is NOTHING under DNS.

This way, it will never work unless you manually type the IP addresses of servers...

On a normal DHCP the DNS furnished by the router would show up there in grey.