MAM accounts do not exist on the Hub, but the subaccounts do,  as the Master accounts are simply a collection of the subaccount, they do not exist on the Hub.

If clients are asking for portal login refer to the CX portal and provide them with a link to where they can log in

Clients shouldn't need to access the Hub for a MM account as no funds are added or withdrawn directly from the account, so anything they need should be available to them via the other platforms.