@Sread 

If the RE450 is configured as a range extender it uses the same password of the router, it can't be different. If the extender is configured as an access point (Ethernet cable from the router to the extender) then the password can be modified. 

Would you be able to provide a screenshot of what you are seeing?

@Tony thanks so much for taking the time to reply. These are the passwords that the tether app has assigned and there is no way of changing them as far as I can see. They are definitely not the password we have for the host router! I have tried logging in with the host router password but only these ones work. 

@Sread 

Rather than using Tether, have you tried factory defaulting the extender (hold the reset pinhole for 10 seconds while plugged in) and setting the extender up by using the WPS method (link) or using your web browser (link)?

What is the model of the main router?

I would also check the firmware version of the extender here: https://www.tp-link.com/support/download/re450/

If you have hardware version 4, then there is not a newer firmware. The one it has already is the latest. If you have hardware versions 1 to 3, compare the build number which is the date to make sure yours is the latest.

Log into the extender with your web browser and you will see the firmware and hardware version on the bottom of the screen.