@AndresMartin Hello, 

Please confirm the Ethernet cable running into the basement could provide Internet first.

Disconnect TL-WA855RE and wire a computer to that Ethernet cable (turn wifi off ), do you have any Internet and what is the IP address on your computer? 

Please reset range extender and set it up in AP mode via Web page ( it is recommended to use a different SSID for RE from main router's)

https://www.tp-link.com/support/faq/1143/

@Drobe  Good day,

May I know do you use Range Extender a wired router or wireless router or modem? May I know the model of your router? 

You could try a different Ethernet cable, better a shorter cable between RE and router when RE work in AP mode. 

If still no luck, can you help confirm some details to locate the issue:

1. wire computer directly to router and make sure router could provide Internet via Ethernet cable,  check the IP address on computer at that moment and share a photo:

https://www.tp-link.com/support/faq/838/

2. Set up RE in to AP mode, connected to Ext Wi-Fi , check the IP address and share a photo

3.  your network topology:  e.g. Router Archer A7 <Ethernet> 855RE set up in Access Point Mode

Does router's LAN Led On when tp-link AP wired to the router? Thank you