As long as you are getting a 3G or 4G signal you can access the internet.
Same thing for wifi, but it needs to be an unsecured wifi signal. If it is a secured wireless network, then:
#1 - You need to know the password. How do you get the password? You need to get from the person's network you are trying to access.
#2 - Routers allows the owner to specify which devices can access the wireless network based on the MAC address (it has nothing to do with Apple). MAC = Media access card. All wireless devices have a MAC address an it is unique to each device. The device's MAC address must be added to the router's list of devices that can access the wireless network. How do you add your MAC address? The owner of the router must do it.