I found it such a hassle to keep locked down I gave up. Like, he'd be so aware that he'd find ways to watch me enter the PIN code when adjusting the settings. I'd have to be ever-vigilant and I got tired of it.
I still need some smartphone for work. Got the smallest one possible so at least games aren't really fun.
In this case, seems pretty topical to just take the phone away entirely for a few days.
Not Dad fiddling with the settings, but instead Dad teaching a lesson about respecting the rules, and doing what you said you would do (children agree to obey the time limits in order to have access to the device).
How Dad should teach the lesson varies by family and by child.