@yaninasona Hello,
For first installation via Tether app, connect to Range Extender's Wi-Fi on your phone and turn off VPN and mobile data on your phone, or Tether app will get the warning that your phone is on a different subnet.
Here is a guide to troubleshoot RE installation step by step, kindly give it a try and let us know if it works for you,
Has connected to TP-Link extender but no internet, what can I do?
https://community.tp-link.com/en/home/forum/topic/198516
For urgent help, you could try to contact our local support from here:
https://www.tp-link.com/support/contact-technical-support/#Hotline-Support