Arduino: Connect UNO to A7 AI Thinker
Without any sketch, I tried to send AT command from serial monitor, and nothing happened.


The problem is with your connections to the Uno. Although it seems wrong, the correct connection when you're using the Uno as a USB-serial adapter to communicate with the A7 AI Thinker module from Serial Monitor is:

Uno                A7
TX (pin 1)   -&gt;    TX
RX (pin 0)   -&gt;    RX


The RX and TX markings on the Uno refer to the pins on the ATmega328P microcontroller. But you're not using that microcontroller, you're communicating between the USB-serial chip on the Uno and the A7. The RX line of the USB-serial chip is connected to the TX pin of the ATmega328P and thus to pin 1, the TX pin of the USB-serial chip is connected to the RX pin of the ATmega328P and thus to pin 0. For this reason the connection between the Uno and A7 shown above are actually an RX-TX, TX-RX connection between the USB-serial chip and the A7.

If you were communicating between the ATmega328P on your Uno and the A7 then the connections you used would have been correct.