I have a Office 365 group that is receiving a warning email that the mailbox is almost full. I know that the maximum size is 50Gb and have set up a Retention policy to delete message older than 90 days

I see that the mailbox is still at the maximum 50Gb which suggests that the Retention Policy we put in place is not working.
The Retention Policy we created deletes message older than 90 days from the Group. I tried creating a policy to delete messages from the mailbox instead of the Group but get an error that says the mailbox is of the type “RemoteGroupMailbox." and cannot be managed in this way.

How do I manage retention/delete content from this Group mailbox?