The fact that they can’t even find it is in part because they didn’t outfit it with any capability to send a distress signal. They lost it multiple times but never added a radio beacon or anything.
It reportedly has radar reflectors and radio, for when it's on the surface.
At 3800 m+ underwater, it'd need to be a powerful beacon. Even most military sonar maxes out around half that.
Additionally, if the problem was a power outage then the radio also might not work unless it had a separate power supply.
Emergency beacons cost a few hundred dollars and are designed for this purpose, not having one is pure negligence.
If they're on the bottom of the ocean then even if they're found a rescue is unlikely to happen in time.
You can buy one for a few hundred bucks, but based on everything I've read about this outfit so far, I'd be surprised if they even had that.