As I read the O.P. statement, I see that he is stating that the release button does not function to allow him to open the bolt, not that he can't open the gun without firing, it seems.
If that is the case, the button (action slide lock) fitting is off and needs to be reworked in order to allow the bolt to be opened to unload an unfired shell, for instance.
I perform comprehensive rework of Model-12 mechanisms and recognize the issue as I have just had another Model-12 with the identical situation recently arrive. Nobody wants to have to fire a shell to open the gun, if it can helped.
A slide lock refit or replacement is not something that all 'smiths can do with familiarity, and I have had to repair botched "fixes" done by those that had no inkling of what the actual functioning part was designed to do.
Most times, the offending repair involves cutting away too much metal so that the button works freely, but the safety function as originally made to work from the factory has been firstname.lastname@example.org