@JPeasey 

Thank you JPeasey, following your link solves my problem (both black and red wire on the same line needed to be added to traveler connection). 

https://automatedhome.party/2020/08/24/replacing-a-single-3-way-switch-with-a-tp-link-hs-210-3-way-smart-switch/

  @gteagle I had the same issue as everyone else and found my solution in Amazon comments/questions. An electrican does a simple walkthrough of how to set this up properly. Tp-Link should work this into their instructions. Has to do with which box comes from the fuse panel and how to figure that out: https://www.amazon.com/ask/answer/MxDLYOLLJYAIZT/

  @jeffthesavage 

thanks for your post. I've installed three sets of 3way switches and only one set wasn't switching on and off correctly. It was very frustrating. I couldn't figure out why that particular one wasn't working until I saw your post. I realized one of the switches weren't wired properly. Once I read your post, I immediately took apart that one switch and rewired. It's now working perfectly. It was really bugging me. Now all solved. Big thanks!

  @Justaguy. 

I am having this exact issue. Did you ever find a solution?

TIA

  @GarrettF Could you clarify what do you mean by "in front of the switch"? I have tried to find the MAC addresses of each switch. Even TP_Link stated that there is no way to find out the last 4 of the Mac. But if you found a spot where this information is visible, you are my savior!

  @GarrettF you only need a smart switch on the first from the breaker box switch .

You can leave your old 3 way on the other end of the leg and it will work. 

Saves a bit of $$