@Lincoln1983 

Hello，

If you change the router's password, then you need to set up the range extender again.

Please refer to the following steps to reconfigure the TL-WA850RE:

Please use a pin to press and hold the reset button for more than 8 seconds to reset the TL-WA850RE to its factory default settings. If you can see all the lights flashing, then go off and some lights go back on slowly, that means the device has been reset successfully.

Then please watch the setup video or follow the instruction to configure it.

Please refer to this FAQ to set up the range extender:
https://www.tp-link.com/en/support/faq/1373/

Note:

Please use your home wifi password to configure the 850RE.
We suggest you customize the network name for the extender. You can see it in step 5.

May it help, and have a great day.