@arvoweirtbnrwti 

Hi, for local management via Tether App, indeed you need to connect the router WiFi first, however, if your router is been bound to your TP-Link ID before and you will be able to use other WiFi or mobile data to manage the router remotely after logging in the Tether App with the same TP-Link ID.

If the remote management doesn't work now or you are not sure if it is bound to any TP-Link ID before, please try the default wireless password first which is printed on the router bottom label called Wireless Password/PIN. If the default wireless password doesn't work, you could connect a computer to the router via Ethernet cable to login the web interface of the router and then find the wireless password. How do I log into the web-based Utility (Management Page) of TP-Link wireless router?

Or you could also reset the router to factory defaults: press and hold the reset button until all LEDs turn on to reset the router to its factory default settings.

After that, you will be able to use the default wireless password to connect the WiFi, and then reconfigure the router from scratch.