The date for the cell phone is only greyed out as a day to pick. It doesn't mean you can't do it that day if you have a date range. Accept it for the day after, read the guidelines and verify it's a date range.
The only thing I can think is there's multiple shops for the same location and someone else is doing the cell phone shop the day you want it.
I dont think Ipsos does multiple shops at one location, I might be wrong. Have you asked the scheduler of your burger shop if you can change the date back or forward a day when the cell shop appears available.?
Have you asked the burger shop scheduler if they can help you get the cell phone shop on the same day?
Asking us can't fix it, but your scheduler might be able to.