This is exactly it!!! Thank you so much, @David-TP!

And for anyone reading this post in the future, with a similar issue, here are the steps I took:

1. I opened the TP-Link app (called Tether). Under "My Devices," I clicked on the RE200. 

Here, I saw the 2.4G and 5G options listed, and the app has generated a new password for them, as David mentioned. Note: it is incredibly long!

3. I copied the 'new' password it had generated.

A few options to do this:

(A) write it down on a piece of paper (click on the grey eye with a strike through symbol to see the password). Again, it is really long!

(B) use the share button on the right, next to the network name, [WiFi name]_EXT, (blue rectangle with an arrow pointing up), and then select the option to copy this text (symbol that looks like two rectangles stacked on top of eachother) -- note, this is the option/symbol that appears on an Android phone so it may look different on an iPhone/iPad. Then, paste this copied text into a notes app (or draft email.. anywhere really), and re-copy only the password (not the full "My WiFi.. Network name.." info). 

4. I entered the new password to connect to the extender, rather than my WiFi password. 

To do this, I opened up my list of WiFi networks on my phone (something like: Settings > Network and Internet > Internet). I selected the [WiFi name]_EXT and pasted in the new password. Repeat for the [WiFi name]_5GEXT option. 

5. Voilà! Connected! 

Thank you again, @David-TP! I really appreciate it :)