my smartthings hub is connected to the internet via a cable.
It may be that this means that my new mini plug from Merros cannot connect to my smartthings hub.
How do I solve this without disconnecting my Ethernet cable?
The Ethernet cable connecting your hub to your router is NOT your problem.
I have SmartThings v2 hubs which connect to my routers using Ethernet cables. I have several Meross plugs and they work fine.
What is the model of your plug and is it standard WiFi or Matter over WiFi? Standard WiFi devices do not utilize your hub. A hub is utilized by Matter devices.
What model hub do you have?
How are your trying to connect you Meross plug?
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and…
Meross Smart Plugs and your mobile device need to be on the same 2.4 ghz network (not 2.4/5 ghz) at the time you are pairing. 
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Install my workflow Matterapperaat.
I only set my router to 2.4ghz.
I reset my plug, then ST sees a new device.
I scan the Matter QR code and connect SM to the Meross plugMSS315.
then I choose the desired WIFI network and enter the password.
Then I get the message 'MS315 is being added to ST.
After a few minutes I get the error code 39-517, ‘cannot find the MSS315 on the network, make sure both are on the same network’
According to my router, Meross is connected.
But I don’t see my ST hub V3 in the network.
How do I get ST into my wireless network?
I’ve had issues onboarding Matter devices to SmartThings with the SmartThings app in my Oppo phone, however with a Samsung tablet it worked doing exactly the same.
Usually I configure the WiFi network in the app of the device and then onboard it, I believe in Meross that’s not an option since it requires to onboard it first to Matter.
I found the solution, I changed the password of my WiFi network some time ago. As a result, the hub was no longer connected to the network.
Thank you all for your thoughts
Wasn’t the hub connected via Ethernet? Why would it matter if you changed the WiFi password? The hub should not connect to WiFi at all if using Ethernet and provisioning of WiFi credentials is done via bluetooth (or other means) from the SmartThings app, not from the hub.