At this time, I'm unable to open user's mailboxes from my Global Administrator account. I'm curious to know if my tenant has an issue with assigning mailbox delegations. I'm aware that user objects must be edited on-premises, but I was under the assumption actual mailbox settings were able to be edited on Exchange Online. If not, then what attributes do I edit to add delegations (SendAs,FullAccess,SendOnBehalf) to mailboxes?
The reason I ask if there is a problem is because when I login to Exchange Online Powershell with the same account credentials, I'm able to user Add-MailboxPermissions on the federated mailboxes with no problem whatso ever. The issue occurs only if I use the EAC. If I'm able to use it on Powershell, shouldn't it work on EAC?