According to these thread posts below by @maddinka you can still access shared mailboxes with Graph and can be just a server permission problem on the server side.
If you do want to go back to EWS then on the desktop go to “Menu / Accounts” and click “Add Account” and “Click the Mail tab” below. Then click “Exchange” and following the wizard.
As far as signatures go “as a Graph account is completely different” you will have to update your signatures. Ultimately “unless you have an On Premises Exchange Server” you will have to change to Graph due to Microsoft shutting down EWS starting in Oct & completing in April 2027
