The main router is AX5400, and both 2.4ghz and 5ghz work.

However, the extender Re605X only works with the 5GHz frequency, and the 2.4GHz frequency seldom flashes on and then remains off.

I have tried resetting the Re605x, and all 4 lights turn on when I push the reset pin. Problem remains the same after I reconnected to the main network.

I have tried disabling smart connect, but some devices only uses 2.4GHz...