That router recovery solution is not suitable for range extender. Try reset the extender several times to see if it will recover

For duplicate "New Device Detected" messages , you can change the position of main deco and the other deco. Like connect the second deco to the modem. Or reset and reconfigure these decos, the issue...

TP-Link doesn't provide support and warranty for the products with a third party firmware. You need to consider it more carefully.

The extender is used to extend the wireless signal, then increase the speed accordingly. It seems the extender is too close to the router. Normally 3 signal bars is good to use the extender. Besides,...

No, extenders can only connect to a wifi with password need.

You can have a try to fix the channel on the wifi extender to 1,6 or 11 and change the channel width to 20MHz or 40MHz. Fix the security type to wpa2-psk, aes. For how to change these parts, refer to...