@Prisoneer 

Hi, the reboot schedule feature on newer routers is designed this way, which is a little bit different from the TL-WR840N, except for the difference you mentioned, the newer routers added another option "every month" which doesn't exist on the TL-WR840N.

May I know why you need to select multiple days for a week to reboot automatically? if necessary, you could selelct Every Day as well.