For office 365 calendars we recommend trying the following:
Please decouple your office 365 email address from all the other accounts first (phone settings > accounts).. You'll then need to go to phone settings > accounts > add calendar > Exchange, as you'll want to add it as an Exchange account. Then sign in with your 365 details. For the domain/server details please see below:
Please go to: https://support.office.com/en-us/article/Set-up-email-on-an-Android-phone-or-tablet-886db551-8dfa-4fd5-b835-f8e532091872#BKMK_O365SetUp
There's a section called: "Set up work or school email accounts that use Office 365"
On the right hand side there's a section called: Use the built-in Android mail app
The only takeaway from the help page is the manual set up, step 4.
Next please see our help page here: https://digibites.zendesk.com/hc/en-us/articles/206762509
If you follow the steps for Outlook.com/Exchange users you'll see there's a part where it asks for manual set up and where you have to fill in email/domain/server etc. In our help page this is for Outlook.com users. You'll need to fill in the information described in the other website's step 4 instead.