Hello  @DrDkny 

During the setup , the mobile phone should be connected to Range extender's WiFi SSID (which has no internet), If the phone connects back to the Wi-Fi of router, the setup can be interrupted. 

1. You could temporarily forget the Router's Wi-Fi network on the phone and try to setup extender again. 

Forget Wi-Fi network :  In phone Wi-Fi settings, for Android: Long press the Wi-Fi SSID and delete it l on IOS  tap the Wi-Fi name and select 'forget this network' 

2. The Configuration Guide here 3 methods to set up the extender, TP-Link Range Extender Setup (Range Extender Mode)

You could also try to set it up on the web page. 

If still the same, can I double-check the model & hardware version of the AC750 extender, can you share a video of the phenomena?