@Mamabear
There might be a sort of solution, but it comes at a cost. It is possible to buy a combination locking router box, i.e. you put the router into the box which is secured to a wall or other fixed object. This will at least prevent your child from resetting the router at their own will. The router power supply can also be isolated by replacing the socket with lockable socket. An absolute faff I get it and not one many people would even consider.
The next part is equally extreme, but once your child has gone to their room, switch the router off completely until the following morning. I accept that in many cases, this might not be possible, but with the router locked away and the power supply isolated with a suitable locking socket, it would prevent tampering once you'd turned in for the night. But then this all depends on you keeping the keys safe and secure. It's all a bit Fort Knox I know.
The problem with all of this of course, is your child is undoubtedly going to deeply resent your actions if you prevent them from accessing the WiFi. But of course you as the parent are supposed to be the one in control, not them. I'm assuming the phone they have is a smartphone and your child has everyday internet access? How is the data for the phone funded? I'm assuming you are the financial tap for this? Can you withdraw that funding or change the phone for one without any internet access at all? Of course none of this is going to prevent your child from attempting to circumvent everything you do by perhaps buying another phone on the sly, a pay as you go data package etc, but that would be expensive and chew data they could probably not afford anyway. The age of your child will clearly have a huge bearing on all of this too.
It's a really difficult position you find yourself in and I hope you can find a solution. I'm reminded however, as a child of the 1970's when there were no mobile phones or computers, life was so much simpler, but here I am almost 50 years on and the world has changed forever.
