@Scorsby,

Unfortunately, the Tether App is only made to work with TP-Link devices. If your router is not a TP-Link router, you will have to use the app provided by your ISP or routers manufacturer. If you have a TP-Link router and it has locked you out, a reset may be the best option to bring your network back to its original state, you will just have to reconfigure your networks name and password

The tether app is meant to connect your Range Extender and help you set up that device. Once connected to the extender, not the router, you should be able to change the Wi-fi settings including your password. 

  @Scorsby,

If it is a TP-Link Archer Router, you can check for mesh compatibility here: https://www.tp-link.com/us/onemesh/product-list/

If you are locked out of the router, a reset is the way forward. If you still have the option to login, you may be able to see a reset password option.

I would recommend setting up the Archer Router with your TP-Link ID so that it can easily be managed from the tether app from outside your network, and then you will login to your router using your TP-Link ID.

Any type of reset to your router will reset the configured settings back to the default login that is listed on the bottom of the router, this will require you to set up the password and name of the network to either a new name or to be the same as the previous setup. If you choose the same network name and password, it is not uncommon to have most of your devices reconnect to the network.