This has been long waited request: getting shared calendars available on mobile device.
An update is currently being rolled out to Exchange Online which will make available on mobile shared calendar.
Even better, the update is coming to support all type of permissions (read, edit or delegate).
All new shared calendar invitation sent and accepted after your tenant is updated will be automatically available on your mobile device; existing shared calendar will not be affected. In this case, just share again the calendar.
This is expected to be completed by the end of September.
Full details available here https://support.office.com/en-us/article/Calendar-sharing-in-Office-365-b576ecc3-0945-4d75-85f1-5efafb8a37b4