macOS Wi-Fi disconnects when Ethernet is connected

1) Try changing the Network Service Order.

Change the order of the network services your Mac uses - Apple Support

2) The USB to Ethernet adapter and/or the other USB cables/devices could be causing interference with the Wi-Fi reception.

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

3) Test the USB-C to Ethernet adapter plugged into the other side of the MacBook Pro.

Ethernet is a higher priority network interface. When Ethernet is plugged in, it becomes the "Default" network interface.

By default the Ethernet port will be assigned a different IP address than the WiFi interface.

If you have a network connection that depends on the IP address, then the other side is going to start seeing your traffic via a new IP address, and it will not be happy.

NOTE: Most web pages do not care about your IP address, unless you are logging in, and even then, they often depend on Cookies to make sure you are authenticated, and not your IP address.

But things like ssh might care a lot if you change your IP address.

It is possible to arrange for the Ethernet and WiFi interfaces to have the same IP address assigned, but it requires either macOS voodoo or router voodoo. But if you manage to get this working, I've done things like start a large file transfer on one WiFi, plug in Ethernet, and see the transfer speed jump up to Ethernet speeds, then unplug Ethernet, and it continues on WiFi at WiFi speeds, etc...

TonyLima wrote:

USB-C with three USB-A ports and ethernet.

Is that an Apple part or some other brand? Is it a single adaptor or some sort of hub or dock?

I run with wired and wireless networks 99% of the time, so it's not an inherent problem with MacOS 26.

When you say 'WiFi disconnects', what do you mean, precisely?

Do you mean you can't access devices on the wireless network?

Do you mean that the WiFI network is disabled?

Do you mean that the WiFi network is enabled, but has no signal bars?

How are you determining this disconnect?

Can you describe your network setup in more detail. Are the wired and wireless networks the same LAN? or different LANs? Is this a managed network? a managed device?

Networks come in all kinds of shapes and sizes, so you're going to need to dig in to your specifics in order to find the cause and solution.

What connection/adaptor are you using for the ethernet?