Did you reset the plug 1 by accident? try to press settings button for 10s to hard reset them and reconfigure them. you may have a look at this instruction: https://www.tp-link.com/us/faq-2229.html

Did you set any mac filtering on the router? Or have you tried resetting the router? recommend to reset the router. If the issue still exists, better to email tp-link support with the model number of...

only you add apple devices, the router's wifi will get disconnection? No matter 2.4G or 5G, both of them drop frequently? set a fixed wireless channel and keep the latest firmware..

We can enable FTP server and access it through ftp:// WAN IP remotely. You may search some instruction from tp-link official website.

As I know, HS210 should work as a pair. so I don't think they can work independently.

Have you updated the firmware successfully? Press the Setting button for 10s to hard reset and reconfigure it.