Hello all, I've got a sewer gas problem in my basement and need some advice on how to prevent it from getting in

The drain is 6" internal diameter inside the pipe..not at the surface. I can take another picture with the measuring tape inside if I need to. It appears to be a clay pipe, but I can test with a magnet to see if it's metal I guess..We have two pvc drain pipes dumping into this drain and every time water is flowing into it, the sewer smell comes out. The pvc drains are from a dump sink and an AC condensate pump so it's just water or sometimes some handsoap being dumped in there. The washing machine drain is connected into this below the concrete as well. There doesn't appear to be any J or P or any kind of trap to prevent sewer gas from coming up. I can't afford to have someone come dig it up and put in a proper trap, so I'm wondering what products are available for this if at all?

I found a SureSeal Inline Floor Drain Trap Sealer but they only go up to 4"! that looks like it would be exactly what I need.

The first picture shows the drain and the two pvc drain pipes



Second picture shows inside the big pipe and the water flow direction

 

If there really is no trap it will have to sealed and vented like any other section of drain piping. I'm afraid I don't have any suggestion on how to do that without breaking concrete. Perhaps one of the others here will know of a way....

 

I'm guessing the only way to know if a trap is further down would be to get a camera down there