Wireless network power switch out of WRT54 router

If you haven’t dd-wrt on your WRT54 router you need to install (flash) it.

Before flashing find which version of WRT54 router you have.
https://en.wikipedia.org/wiki/Linksys_WRT54G_series
Version 5 and above switched to VxWorks OS and reduced flash memory and RAM, that is much more complicated to flash.
It is recommended to have a version under 5. also because new dd-wrt build missing ssh service.

Find appropriate flash-file and on the same page you have link to instructions.
https://www.dd-wrt.com/site/support/router-database

Follow instructions carefully word to word or otherwise you brick your router.
To use tftp on debian based Linux, install terminal application sudo apt install tftp
I have version 6 to be flashed: https://dd-wrt.com/wiki/index.php/Linksys_WRT54GS_v5.0_&_5.1_&_6.0

After you have dd-wrt flashed in you can configure router.

Reset router.
Connect to router on browser in address 192.168.1.1.
Setup Router Username name and Router Password.

Wireless > Basic settings:
Wireless Mode = Client
Wireless Network Name (SSID) = Your present wireless network name.
Wireless Channel = Auto
Save.

Wireless > Wireless Security:
Change settings to match your wireless network security.
Save.

Setup > Basic Setup:
Local IP Address = Should be other network like 192.168.2.1
Subnet Mask = 255.255.255.0
Gateway = Your wireless network router IP address
Save.

Administration > Management > Remote Access:
Telnet Management = Enable
SSH Management = Enable
Change port to something else, not default.
Allow Any Remote IP = Disable
Allowed Remote IP Range = Your wireless network IP range
Save.
Reboot.

More about dd-wrt gpios and funtions
https://www.dd-wrt.com/wiki/index.php/LED_Scripts#Linksys_WRT54G

When amber color LED is turned on your’re connected to the wireless network.

Connect via terminal using telnet and test (gpio 6 will reset router!)
Attention! Telnet is UNSECURE connection type and this is only for testing purposes in home LAN network.
gpio disable 2 will turn on Cisco button white LED
gpio enable 2 will turn off Cisco button white LED

Connected 5v relay module which seems to work also with 3.3v.
Telnet command gpio disable 7 will turn relay on which is connected to router LED output LEDC7.

For testing purposes only as Telnet is not recommended to use in any remote purposes. I bought very old wrt54 with no SSH connection deployed in that version of dd-wrt.
On the left side video shows relay connected to LEDC7 pin on WRT54 board. When we type in terminal right side gpio disable 7 it enables relay and green status LED is on.