Use a voltmeter , measure voltage on line when nothing runs,
measure with router on, measure with both on.
If it goes down more than about 10 percent, you got a too long,
or too thin power line.
say 100% voltage is 100 % power on router
then 90 % voltage will give 81 % power at router.
You can see it changes fast.
P (power)= U x U ( U= voltage) So it is U at power of 2 divided by resistor value.
It is right for pure resistors like a calorific heater, but It' s an aproximation, because with an universal motor, the power reduction is worse than that.
so if U= 1 (100%) then U to a power of 2 is U x U = 1 x 1 = 1 (100 %)
If U= 0.9 ( 90%) then U to a power of 2 is U x U = 0.9 x 0.9 = 0.81 ( 81%)
That shows how fast a 10 % voltage drop gives a 20 % power loss.