What's the model number of your router? Is it TL-WR841N V11?

AC2300 supports wireless mac filtering. Put your devices to the white list, and other wireless devices can be blocked.

Sorry to hear that. Archer C8 shouldn't have so slow speed. Perhaps TP-Link can provide you another solution.

Did you upgrade the firmware version of your plug and also the version of the Kasa App and Alexa App?

Are you able to configure the plug fine when the ssid is visible? Actually this won't work after you hide the ssid again. When you hide the ssid, pay attention to the security type you choose on the...

Move the plugs close to the router to have a check. if reset and re installation can't help, better to contact support.usa@tp-link.com for help. It is not a common issue.