@Dam340 

Hi,

First, a brand new RE450 doesn't have any password. It will only have passwords after you've connected it to a Wi-Fi network or set a password by yourself.
If your RE450 currently does have a password set and you don't know what it is, then reset the RE450 to factory default settings via the small recessed "Reset" button and any password(s) will be cleared.

TP-Link offers three methods to connect their range extenders to a Wi-Fi network.
1) by using the "Tether" app
2) via the extender's web management interface (requires a web browser)
3) the "WPS" method

Since the Hotspot feature of the iPhone likely doesn't have a WPS "button", you can't use method 3).

For 1) and 2) you will need an additional client device, because the one iPhone you use as your Internet hotspot can't work as a "router" (which is what the hotspot feature basically is) and client at the same time.

So you will need to find another client device you can use. Anything that has Wi-Fi and a web browser would do. (older smartphone, tablet, computer, game console, even a somewhat modern TV should work)