Hello @JeffL 

Sorry for the late reply, can you share the Mac address & hardware version of that HS210 to us via private message?

Can you help describe how you notice the switch is factory reset? Did you try to press the ' Reset button' for 10 secs to factory it? 

Do you have HS210 with the same hardware version work fine in the same network? Can I know the model of router, thank you

Hello @JeffL 

Thank you for providing the case details in pm.

I did not notice that even manually HS210 can't turn on the light, if so, please pull the switch out of wall box and checking the wiring of HS210 first.  

1. Ensure the wires are still properly connected, not getting loose (cut off breaker and test with a voltage tester before tug the wires)

Here is a set-up video to examine the wiring of HS210:
https://www.tp-link.com/support/download/hs210-kit/

2. If you have more HS210 , try to replace the problematic HS210.

May I know how about the LED on 2 * HS210, WiFi led and White Circle. Do you use them as a KIT or with an ordinary 3 way switch?

If you ensure the wires are properly connected, and manually smart switch still can not turn on the light, as the switch has been working fine for a few months, it could be a faulty unit,  I recommend to contact retailer or local support to get it replaced under warranty.

Hello @JeffL 

Thank you sharing and updating here and let us know!

Hope everything goes well while get this unit replaced.

Merry Christmas!