Remove all sensors does not do the same as remove sensor, it simply purges all sensors from the Almond+, it doesn't reset them.
I highly doubt the router is defective as we've had zero Z-Wave issues.
Z-Wave has some quirks and I've had problems adding certain types of sensors as well.
According to the manual it shouldn't be that hard to join the sensor though, as it simply states "When prompted by your primary controller, tap the top or
bottom of the rocker" which I guess is what you've said already. Normally you have to switch it on rather than off for it to join the network.
One question, does it switch on and off the light it's connected to?