That's not what I said. Using a vpn you can tell if it is isp locked or ip locked.Yes we know that but at least we can suggest things for him to try and see. Saying you don't need to do any of that and just use a VPN isn't the right call is it?
So if his dad's sub works you login with that then turn on vpn if it stops working you know its ip or isp locked
But we know that's not the issue reading through post. As he has tried his dad's and its working.