@Lennard 

If you have mobile data you should be able to log into your router through the tether app and adjust your devices, 

Just tried this with my mobile and I was able to log into my router via tether app and can make changes using mobile data. 

Once on the tether app I would suggest creating profiles for each of your family members. 

That way you can implement profile rules per person for each person's devices. 

Bear in mind if you have a laptop or xbox ect then to be sure to add both ip addresses of each device if the means are there  for wired or Wireless for that device as if you want to make sure your child can't get around the no Internet off time as each means of connectivity will give a different ip adress. 

Also if say your child has a mobile phone or tablet/pc laptop and they install a vpn and activate it before the Internet time is up they will still be able to access the Internet as well as any blocked websites including adult websites whilst the vpn is active. 

To put a blanket stop on all vpn sites as long as it has vpn in it on the page where it says block website on the profile enter vpn dot com without the space then save it, 

So if the try entering onto a Web page with those words in it, it should put a blanket ban on any website containing those words and refuse to load. 

I think you have a limit of 32 websites you can block on the block page so try to keep the wording short but to include the main wording to apply to multiple website names. 

Hope this helps