@Gary.c-9
Theoretically, you do not have to reconfigure it. It will work with it plugged back to the wall socket after the initial setup.
We will check some details to do further analysis.
1. What is the led status of the TL-WA850RE after the initial setup? Please use the main router's wifi password to configure it.
2. What about it since you unplug and plug it back?
3. After you plugged it back in, please login to the web UI of the TL-WA850RE and show us a screenshot of the status page.
4. Besides, please ensure that the firmware is up to date. And what is the model number of the main router?
Good day.