Office 365 shared mailboxes do not require a license so autodiscover and ActiveSync will not work and the mailbox will not be accessible on a mobile device.
There are two ways to get around this problem:
1) Re-create the mailbox
Backup, delete and re-create the shared mailbox as regular licensed account. Restore your emails from backup and setup appropriate permissions.
2) Convert shared mailbox to regular mailbox
You will need Windows PowerShell to do this so install form below link if required:
1) Launch Windows PowerShell (Run as Administrator)
****If this is your first time connecting to O365 from your computer, please run the following command first before proceeding to step 2:
The execution policy helps protect you from scripts that you do not trust. But we need it to be unrestricted so we can run commands. Enter “Y” and proceed to step 2 below.
2) Enter command
$cred = Get-Credential
Enter the credentials of an administrator account in the O365 Company to login.
3) Issue the following command to create a connection.
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $cred -Authentication Basic -AllowRedirection
4) Create session
Finally, this is the command you need to execute to convert the mailbox from shared to regular, replace email@example.com with the email address
Set-Mailbox firstname.lastname@example.org -Type Regular
After running the above command and modified the mailbox as Regular, go in the admin section, reset the credential of the user. Login to your mobile device now with full email address.