Help! Gas tank/fuel pump retaining ring not screwing on

Hi everyone....so I have an 05 rx8 that's been in the shop to have the fuel pump replaced. Now that they are trying to screw the retaining ring on they told me it won't screw on. They said the black ring with the threads on it that is attached to the gas tank is broken in half (see picture).....they're telling me the whole gas tank needs to be replaced because that part comes automatically attached and you can not reattach it. I've tried to look online but can't find anything or even figure out what that part is called....can anyone shed some insight into this for me and what me next steps should be? I apologize in advance for not knowing the names of everything. The first picture is to show the black cracked ring that the retaining ring needs to screw onto, second is a picture of the pump in the car but the ring not screwing on without that black piece being there.....thank you so much.