You're fighting a losing battle trying to use technology to control teenager's internet access. They're better informed than you about technology, they talk to their mates about how to solve technology problems, and biologically they're becoming argumentative young adults. If they want the access badly enough they'll get it, unless you can physically deny them access to technology (like take their phones off them "out of hours", and having bullet proof control of your router), but you'll also need to make sure they've got no money. An old, cheap, smartphone can be had for twenty quid, and a PAYG SIM topped up with cash, offering direct access or a low cost wifi hot spot.
Talk to them (one at a time) don't get into an argument, because in that case you've already lost, explain your concerns, LISTEN to their views, and offer some compromise. Bear in mind as well that older kids will progressively be developing a normal interest in shall we say "adult content". You might not like it, you'll have to either become the repressive Victorian parent, or accept that they may be surfing the net for *ahem*. That can be an embarrassing discussion, but better to let them know what's illegal, what's dangerous, and that although you don't really want to know, you accept they may have an interest in grown up matters.
