On:

echo 1 > /sys/class/gpio/gpioN/value
Off:

echo 0 > /sys/class/gpio/gpioN/value
Get current state:

cat /sys/class/gpio/gpioN/value
Here, N should be replaced with pin number, which depends on router model. Here are some known pin numbers:

Model	Pin number(s)
TP-Link TL-WR703N	8
TP-Link TL-WR842ND	6
TP-Link TL-WDR3600	21, 22
TP-Link TL-WDR4300	21, 22
TP-Link TL-WR1043ND	21