@FastBack 

It is strange that the 2 TD-W9970 have the same issue, we would like to do some troubleshooting and try to fix it.

1. What is the light status of the TD-W9970 when it goes offline?

2. With your computer or smart phone connected to it at that time, what is the IP address and default gateway it obtained?
Note: How to get them: https://www.tp-link.com/support/faq/838/

3. Meanwhile, please try to access the web page and take a full snapshot of the status page. 
Note: How to login to the web interface: https://www.tp-link.com/support/faq/114/

4. How do you make it back to work after it goes offline?

5. Furthermore, when did you buy it?

6. Here is an instruction you can follow to troubleshoot as well.
https://www.tp-link.com/support/faq/2238/

Look forward to your reply.

Have a nice day.