@afarber 

Hello, 

How did you upgrade the firmware？ By wireless or wired connection?

You can connect your computer to the range extender via Ethernet cable then use tplinkrepeater.net to access the web UI.

Here is a FAQ you can refer to reset the range extender, please note that if you can see all the lights blinking once on the RE200:

https://www.tp-link.com/en/support/faq/202/

Here is a FAQ you can refer to do the troubleshooting if you can't access the web UI of the RE200:

https://www.tp-link.com/en/support/faq/673/

From your reply, the Internet of RE200 works well, right?

What settings did you want to change on the web UI?

If you want to change settings of the RE200, you can also download the Tether app from APP store/ Google play on your cell phone. Then check if you can see RE200 on the Tether app.

If you need any further help, please tell me what error information you have got when you access the tplinkrepeater.net. May I have a screenshot of this page?

Have a nice day!