Your issue is not related to the DHCP lease time. Change the channel which is less used by other wireless routers. You can download inssider to check which channel is better. Change channel width to...

Actually you can go to Advanced- Wireless-Wireless settings to uncheck Enable Wireless Radio of the band you don't want to use. But first, you need to login to the modem's page via 192.168.1.1.

It will take some times for them to reply you. How do you know the router is rebooting?

Was the USB light on when the hard driver disconnected? If the external hard driver has an external power supply, better to use that power supply to power for it instead of the usb port. Besides,...

There is a mail box icon on the top right of Kasa. You can see some notification there.

Are the deco running the latest firmware and all connected to the same router or switch directly?