Yagi antenna is very directional. If it is well made and if impedance matches between router and antenna there'll be no harm or you can even damage the router radio chip. And long cable also causes loss. If you want to use it on one direction only for a device, why don't you try? This kind of gain antenna improves on both RX and TX. Let say you have the antenna with 6 db gain but if calbe to connect it is long, lose may be 1 db or higher. So 6 db -(loss in db) = actual gain. Only way to know is to try.
