@Fellioe 

Hi, for your case, here are some suggestions. 
1. Please check whether Wireless password of your new router you entered is correct or not.

Please go to Settings--WiFi on your phone to forget the new router WiFi, then reconnect it using the same Wi-Fi password to see if it works. 
2. Please reset the extender into default settings again by holding the reset button until all lights flashing once, then refer to this instruction to configure your range extender again carefully:

Via Tether App: https://www.tp-link.com/support/faq/1373/
Via Web interface: https://www.tp-link.com/support/faq/1400/

Note: During the configuration, please make sure that the extender is close to your router and be careful when you enter router's wireless password.  

If it still doesn't connect, please check if there is any special settings on your main router, like access control etc. which will block the extender.

If no such special settings, please let us know the following information:

1) model no., hardware version and the LED status of your range extender
2) distance between range extender and your router
3) model number and hardware version of your main router