In parental control, when you set limit for specified devices, it is enough to restart router and the used limit starts from 0. So clever child (like mine) will get unlimited time for internet surfing. Can't used time be stored in non volatile memory at least every 30 minutes. I know that flash memory has limited write cycles and needs to be used carefully but maybe there is another way, e.g. store it in the cloud or something.
Parental control - browse time limitation
Model: Archer C6
Hardware Version: V2
Firmware Version: 1.3.6 Build 20200902 rel. 65591