If they have an EBS ( emergency breathing system) in place they can survive the 96 hours of breathing through that.
But if it has no escape hatch I've no idea how they can be rescued by a DSRV.
The only way would be to pick it up and transport it to the surface but that I think is impossible...
The CO2 will slowly kill them ,( they will just drift off to a permanent sleep )not the lack of Oxygen.
It does seem strange as they must not be able to blow their ballast tanks either or drop any weights they might have been carrying to have the ability to submerge in the first place ???