@NikosG 

Hello, if you don't want to use RE200's Wi-Fi but only want to get internet from the Ethernet port on the extender, you can try to disable extender's 2.4GHz and 5GHz wireless function to check if it works.

But I need to warn you that the extender should still connect the router by wireless connection. If you turn off the extender's wireless connection for both band, we can't make sure if it will 100% work. Since you didn't provide the hardware version of the extender, I can't test it in our lab. You can test it by yourself.

 You can enter the management page to set it up first, then go to Settings-Wireless-Extended Network-Disable extended 2.4GHz and 5GHz-Save it.

Then, the wireless function of the extender is turned off. Please check the lights' status on the extender and if you can get internet on your PC when it is connected to the Ethernet port on the extender.

How to set it up: https://www.tp-link.com/support/faq/1400/

We also have a better suggestion, you can try to Hide SSID broadcast on both bands.

Note: ' Hide SSID broadcast' is at the same page as 'disable Extended 2.4GHz/5GHz network'.

Once you click on 'Hide SSID broadcast', all the wireless clients can't see the extender's Wi-Fi name.

If you want to connect the extender's network, you need to input the totally correct Wi-Fi name and Wi-Fi password from the extender to connect.

Hope this answer can help you!

Good day!

@NikosG 

Good day.
Thank you very much for your time and patience.
After checking with the senior engineers, it turned out that at least one wifi band needed to be enabled in order to communicate with main router to get internet service.

And if you only want to have one wired device to get interent, you could try to set up the access control>enable the white list and only add the wired computer;

Thank you~