you can try WiFi Clone as the user guide.

try high-speed mode: router use 2.4G only and RE use 5G only(disable other band in router and RE). RE use half-duplex mode so the speed will be halved.

make sure you haven't got a loop firstly. And how about the LED, could you log in the web management when you drop connection.

How about enable the DHCP server?

yep, you have to setup them one by one

Did this happen in local? If you just cannot see the live view remotely sometimes, but the notification is normal, I suppose that may be caused by the outside internet, such as bandwidth or rate.