You won't burn down your house, but it will likely shorten the lifespan of your router, depending on how it's internals work. I can't say for sure, but I would bet that there are internal voltage regulating components that convert the 9 volts down to 1.5 or so for the processor to use. If it hasn't broken yet and it's not too hot, then you'll likely be fine, but I definitely don't recommend it.