Other than turning off the wifi which is a pretty blunt instrument and will also affect every other wifi device in the premises, i dont think the Now Router supports timed wifi access based on the device.
Other routers do however support it - each device has its own Mac address and on routers that do support it you can then select when they can ( or sometimes can not) access the internet, its also possible to manually block devices from connecting all together until you put them back on.
For example I use a TP link router ( model Archer VR2800) and the parental/ access controls looks like this screenshot below, select scan and it will list devices select the device and then give it a time it can /can not connect
Its also possible to use the phone app to block certain devices from connecting, and then allow them to connect when you turn it back on ( phone need to be on same wifi network)
AS ive mentioned before i can only speak about using a tp link router - other makes/models are available , i purchased my router and archer VR2800 on a special offer , other routers will do the same for a much cheaper price than the VR2800 and if you are thinking of going down the own router route then you need to be aware that not all routers will work with Now or Sky.
if you need help with tp link routers see here: https://community.tp-link.com/en/home/forum/121
Now will not give any telephone support for a third party router should you have issues with the connection - if you end up in that position you will need to plug the now router in until the issues are resolved do not dispose of, or think of getting rid of the now router - keep it to hand just in case
As well as TP link, other manufactures worth looking into are Asus, Billion, Linksys amongst others
further reading : https://helpforum.sky.com/t5/Broadband/Sky-MER-Compatible-Modem-Router/td-p/3852647
