Because you took possession of it and agreed to pay rent for it. When you "rent" something, you take possession and agree to pay the rent until your agreement is up and you return it. You act like they're doing something to you, but you're the one who rented it. If you didn't want to rent it, then you shouldn't have rented it, or if you didn't want to keep renting it, then you shouldn't have kept renting it.
You know, when you go to a restaurant, you can say the food is bad and send it back. But if you eat the food, you can't expect to get way with, "Well, it wasn't very good, so I'm not going to pay for it." That's what you're doing. You're saying, "Well, the router sucks, so I shouldn't have to pay the rent." You eat the steak; you pay for the steak.