I'm leaving from Porto tomorrow morning and I got my credential today at Rua Das Flores. I don't know the exact housnumber but it's a big yellow house on a corner. It's an official building with lot's of different offices and an official looking plaque on the front door.
The camino office is on the first floor Office number 14.
However it doesn't look like it has specific visiting hours so maybe it's better if you call first at number: 00351/91.838.73.05
it's the number that's posted on the door of the office and it connects you with the volunteer that works there.
I don't know the opening times at Porto cathedral. But I got my credential on a Saturday at 11 am with no problem. The Cathedral is also a tourist attraction so I guess the information desk is open all/every day more or less.