Just use simple properties.
var tomorrow = currentDateTime.Date + 1;
var eventsCustom = eventCustomRepository.FindAllEventsCustomByUniqueStudentReference(userDevice.UniqueStudentReference)
.Where(x => x.DateTimeStart >= currentDateTime.Date
and x.DateTimeStart < tomorrow);
If future dates are not possible in your app, then >= x.DateTimeStart >= currentDateTime.Date is sufficient.
if you have more complex date comparisons, then check Canonical functions and if you have EF6+ DB functions
More Generally - For people searching for issues Supported Linq methods in EF can explain similar issues with linq statements that work on Memory base Lists but not in EF.