guest user admin

Taen keren 
Sorry to pull this one out of the grave, but I came across this by chance, and thought I would just fill out the answer for anyone else passing by 🙂

The thing is that a guest identity is not really the best way to access these other portals - MS is working on it, but this work is mostly relevant for Partners that have adopted the partner identity eco system.

IF you are "just" helping out a few clients and have never bothered to setup your MS partner account for management of clients, then you would be best off by creating a dedicated user in the clients tenant.
There is nothing "wrong" in doing this, but you might not enjoy having to manage too many separate identities though.

Hope this helps.

More on MS Partner delegated access here: https://docs.microsoft.com/en-us/partner-center/customers-revoke-admin-privileges