This may be related to the use of VPNs on your kid's device(s). There may be software that can "spoof" the MAC address of the device connecting, thereby registering it in the gateway as a "new" device with no assignment to the child's profile.
I have had a lot of similar issues in my home. The solution depends on the age of the kid in question and who is paying for the devices. In all fairness, if you bought the device and pay for the cell plan then the devise is yours to do with as you want and the child in question is borrowing it from you. If that is the case, you have some options that can be very effective.
If that is not the case, there are still some solutions.
Most routers have an option to "block unknown connections". If you activate that feature it will likely put an end to "spoofing", but may cause other issues with (for example) your anti-virus software, or other connections Windows may make to privately do updates etc.
I would give it a try and see what happens.
A very effective solution I found (due to the objectionable content that was being accessed on the child's device) was to have a talk with the child about their device usage, and about reasonable boundaries for use, and about the fact that one day they will work for a company that will monitor every aspect of their computer and phone use, so they should get used to accountability. I informed the kids that since the phones and laptops were all owned by me, that I was going to install monitoring software on both of them just like they will face at work in the future. I proceeded to download and install Qustodio software on both devices, and things got a lot better. I also used Open DNS as the assigned DNS on the router to help block some of the evil junk on the internet, and to prevent access to VPNs etc. I used Qustodio to block all VPN use on the device itself and uninstalled all of that sort of stuff from their devices. I also blocked their access to our anti-virus software using Qustodio because they discovered a built-in VPN withing the anti-virus to get around Qustodio. It's definitely a bit of a cat and mouse game but your kids' long term wellbeing is worth it.
For a while, we took physical possession of the devices every night as well just to be sure.
One thing I like about Qustodio is that its on the device so they can't just get around it by going to another network or to a friend's house. Secondly, you have a lot of control to be able to gradually allow more privileges as they demonstrate their ability to show self control.
There was a lot of complaining at first, but I proved to be very reasonable and fair about things, and soon they realized that I was only helping them get more schoolwork done and improve their grades, which would help them. I had to get good at being responsive to their needs for school website access and at having discussions about the sorts of web activity they were having, helping them understand how they were using it as a crutch in life rather than face issues, and help them realize more successful approaches.
The success story is that both kids now are in university on scholarships and have reasonably responsible internet usage habits.
The main thing is to have accountability and reasonable expectations all the way around--both you and them, and to work as a team to solve the problem of their overuse or misuse of the internet.
Internet is an addictive drug, and kids need to learn boundaries and how to function effectively within them.
