I am assuming that you have changed the Routers default password? This can be found on the underside of the said router. If you have and you cannot remember your password then you will need to reset the router by whatever method your router uses. Mine for instant need a paper clip press in a small hole for 30 seconds or some such time. Yours may be different. Your manual will tell you.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been