I went through it and circumvented it completely in the 90s when the only way online was a computer with a modem in the living room. It's so much easier today, its absolutely trivial to circumvent anything you're doing. Old smartphones from "a friend's brother" are easily hidden and can be used on wifi you don't control.

All it takes is the kid wanting to go behind your back, the rest becomes easy for them. The only chance you have is establishing a good relationship with your kid and instilling good values. You can't actually control them online unless you lock down their life like a supermax prison.