Having been an executor twice in my life, I can sympathise with you.
Many instituitions don’t recognise that people die. It makes it extremely hard for the successors to then transact business on behalf of the deceased.
All I can advise you to do is obtain a copy of the death certificate and a copy of the probate . You’ll probably need both as originals and attested to as originals . With these you can approach the instituitions to have the executor added to the persons able to transact.
It’s not likely that anyone but the executor can be added as a transactor, but once thats done the executor can then transfer that right to someone else.
I wasted many hours navigating this path with many instituitions.