I already have a modem do I have to buy a new one, if I change my internet or WiFi providers?
it depends if ur internet is provided through a cable or phone company. if it is through a phone company you need to make sure it will work on that specific technology
If it's like a Belkin one, than you shouldn't need to.
If you switch internet providers and previous company owned modem they might want it back
nope nope nopidy nope