@DavidRoth 

Hi, what is the model number and hardware version of your TP-Link device? you could check it on the bottom label.

Except for the new phone, do you have any other client devices connected to the WiFi or connected to the router via Ethernet cable? if so, you could login the web interface of the router, then go to Basic->Wireless part to find the WiFi password.

If it is a 4G LTE router, please refer to the instruction below to login the web interface:

https://www.tp-link.com/en/support/faq/1656/

However, if no any client devices can connect to the router, you could try the default wireless password printed on the label of the router itself, if it doesn't work, please reset the router to factory defaults: with it powered on, press and hold the reset button until power LED starts flashing, then release it and wait it rebooting and back on.

After that, you could connect to the default WiFi with default WiFi password, then reconfigure the router.