If you used an immigration lawyer to apply for permanent residency, you can't use the portal to make those updates, because the original visa is not associated to your portal account.
So your only other option besides emailing the form, is to print it and bring it in person to an office.
I know this, because this was my situation until recently.
A few months ago I changed my PR subclass and the new application was done using the portal. Which means that I can finally use the portal to update my details.