Here it is! 

I have a guest network but it does not have schedule option.

I see you are on the newer Green User Interface as I am on the old Blue User Interface. The Green Interface lost some functionality on my C8 so I went back to the Blue one. The Blue interface on my C8 has guest Access Controls. 

Try this: check whitelist but not add any keyword. This is solution from other treat by tplink guy. The logic is that will be used whitelist policy,but when list is empty all traffic will be blocked. There are many questions about this settings in all tplink router.

Tried, no success. I am totally disappointed on this router. It does not even do what my old 5 years router used to do. To me parental control is crucial and this device fails on this. Waste of money.

@ArcherC8 I have identified another vulnerability of the router related to the Parental Control. My son is smart enough that started avoiding the time limit was assigned to his devices him to concentrate more on education. The solution is very simple, he restarts the router after which it forgets the time spent by user and stars counting internet access from 0. More I use this device more I believe that my money wasted.