Remote control an Arduino that is out of sight

I have a simple arduino system set up at around 50m away that is connected to an external power supply and it is out of sight. I would like to be able to switch the system on and off remotely. How could I do this?

I thought about using a second board and connect the two with RF transmitters and receivers, RF modules are not locally available where I am from. Are there any alternatives?

Once the remote Arduino is turned off, then as it will not be running so how will it be able to receive a command to turn it back on ?

If there is power at the remote end, as you have said, you can use a transceiver such as the nrf24L01. The receiver would be at the "out of sight" end and would be permanently powered, ready to respond to a signal from the transmitter, say switching on and off various circuits etc.

If the remote end is battery powered only, or there are other reasons you wish to preserve power, then you have to look at other solutions.

Perhaps put your remote Arduino into deep sleep.
https://www.youtube.com/watch?v=P4Em7H0vtsU

Why?

Given that it is

Depends what “ system” means - maybe he wants to switch on /off whatever is connected to the Arduino ?
Can the OP explain the project pls ?