I just had the same problem in a 1 year old house. We had an unbelievable smell. It took 6 weeks and about a dozen people to find, but it’s finally fixed. It was a 2 part problem. One of the problems was the wax rings around the toilets. We had them changed, but the plumber that put the new one in upstairs should have used 2 rings. We have a tile floor and a cement subfloor so 1 wax ring wasn’t high enough, therefore not doing any good. Then we had the plumber do a pressure test, which lead him to find 4 holes in our venting system. This test seemed easy enough to do if you have a good plumber. After fixing all holes and once again changing the wax seals, using spacers this time, our smell is gone. Maybe you could check these out. Hope I helped! Good luck!