Looks like those are to attach sockets to a chassis back panel, yes?
First, use cyanoacrylate glue (aka Superglue), the thin type not the gel type. Apply sparingly into each crack, and use a C-clamp to hold it together until it dries. This is just to return the piece to the original dimensions without extra play where the screw, screws in.
Next get a round piece of plastic of larger diameter, for example the body of a plastic marker, or a piece of PCV plumbing pipe, etc. with the end of it plugged.
You want to slide that over the cracked piece, as a shell around it with an air-gap in the middle. The end of the shell should be flush with the end of the cracked piece, where it screws to the chassis. You will seal the end with masking tape to form a leak-proof barrier. The other end of the shell should be plugged so it is also a leak-proof barrier. The bottom of the shell should be taped to the cracked piece, also forming a leak-proof barrier.
The top of the shell should have a large hole or a split portion. You are essentially building a tube around the cracked piece with the only way for liquid to get in or out is through the top from gravity.
Into the large hole or split portion, fill it with slow set epoxy.
Another alternative is put in a very thin layer of epoxy then lay fiberglass cloth around the perimeter, coat that with more epoxy, through another layer of fiberglass mating on top, and coat that with epoxy.
Another more permanent (can't be disassembled again later without much frustration) is reassemble the whole thing into the chassis but just before you screw the screw into the cracked portion, fill the whole with epoxy then screw the screw in, then let it set for a day.
Another option, forget about those PCB mounted jacks. Get panel mounted jacks, mount them to the chassis, then connect them to the PCB with wires.
Another option, make a back-plate that sits behind that vertical jack assembly and extends past both sides of it. Put two additional holes in the chassis wall and run long screws through the chassis holes, into the backing plate behind the jacks.
Another option, leave those jacks there but unused. Run jumper wires from their solder points on the PCB to a different location where you mount panel mount jacks in new holes you drilled into the wall of the chassis.
Another option, if the outer barrel of the jacks is at chassis ground potential and the chassis is metal, solder them to the chassis wall, or if there is clearance to still get the jacks inserted, after protruding from the chassis wall, slip thin washers over the sockets and solder those to the socket outer barrel metal.
Another option, use a very long bolt and put the bolt all the way through the cracked piece and secure the bolt behind the cracked sleeve with a nut.
I'm sure there are other methods but surely one of the above will work.