The animal is dead. You don't smell fish right away. They do not abandon shell when moulting. if it was moulting it would be in the shell and you would see the shed skin lying there ( which some people mistake for a body of a crab)
They ditch their shells when they are stressed or sick.
If you're keeping it in a plastic cage then it likely cannot get the right amount of humidity causing it to slowly suffocate. And plastic tanks cannot be heated safely so it may not have been in the right temperature.
Or the damage may have been done prior to the move to the plastic tank. common issues could be- exposure to bad food, painted shells damaging gills, chlorinated water damaging gills, no ocean water access, mites, or damage caused in a pet shop.
Stress can be caused by living isolated, and living in unhealthy conditions.