New M1 iMac switches from ethernet to wifi

The original post link is here:

https://support.apple.com/en-hk/guide/mac-help/mh11939/mac

1. In many cases, you’ll automatically connect to the internet. If not, complete the following steps to enter your network settings. If you aren’t sure what to enter, check with your network administrator or ISP.
2. On your Mac, choose Apple menu  > System Preferences, then click Network.
3. Select Ethernet in the list on the left.
4. Click the Configure IPv4 pop-up menu, then choose your ISP’s recommended configuration method.

• • Using DHCP: Choose this if you receive an IP address automatically from your ISP.
  • Using DHCP with manual address: Choose this if you received a specific IP address and your ISP uses DHCP or other network settings.
  • Using BootP: Choose this if your ISP uses BootP.
  • Manually: Choose this if you received a specific IP address, a subnet mask and a router address from your ISP.

1. Click Advanced, click DNS, click the Add button  below DNS Servers, then enter the IP address of your ISP’s DNS server.
2. If your ISP gave you to search domain addresses, enter them as well. These may not be needed if you configure your IP settings to use DHCP.
3. If you were given other settings from your service providers, such as IPv6, a proxy server or additional Ethernet settings, click Advanced to enter them.
4. Click OK to make the settings active for the Ethernet service.

Check the service order to make sure Ethernet is on top.

1. open your iMac's network preference
2. click on the cog (bottom left)
3. select Set Service Order
4. drag Ethernet to top
5. then click OK

I apologize for using black on white, but it's easier on my eyes than white on black.

No it is not. Which brings up the next question.

What is the CAT rating of the cable between your Router and Mac Mini?

If it is an old Cat 5 or 5e cable, then you need to replace it with a Cat 6 or Cat 6a cable.

Could it be an issue related to your speed test? I would leave it on unless you see it surface some other way.

😉👍

Comcast is messin’ around…but they’re clearly not very good at it 😁

Good to see your problem resolved.

Yes

See my post 4th Sept 2021 and helpful reply

Rob