There is currently no support for "day splitting". It is on our list for future development.
If all your shops have 9-19h opening hours, the work-around would be to plan one route, without any time slots and without multiple rounds.The idea is that you drive until the shops close, then you find a hotel, and continue the next day. Download our CSV file and import into Excel to calculate arrival times, if you need these.
The maximum round time is a soft restriction too. If you plan multiple rounds it will try to keep the duration of all rounds, but it will not create additional rounds for you if it does not fit.