Not sure if you have sorted out your problem.
I am fairly certain that the same model of safe was used throughout the site, as long as that is the case then you can open the safe when the batteries are dead. When you purchased the apartment, you would have been provided with details of the safe including instruction leaflets in Spanish & English. Those should advise you what to do when the batteries are flat. I dont have my documents with me but will be in DV next Friday if you still need help sorting. I think, but am not certain, that you will need one of the little 9v batteries. You then hold this against the door in a certain position, look for + - marks on the front, you will of course still need one of the other keys, plus the code etc to open.
I know there is an agent for the safe manufacturer in San Pedro and his number was included in the pack provided originally when the apartment was 1st sold. If you can find any other owners out in DV at present they may be able to help you with details if you have not yet resolved the issue.
John