Did you chxk the seal on the toylet?
Sometimes the wax is bad.
If the wax is sufficient, there should be no sewer gas. I like to feel the wax squishing down as I push the toilet down. Or it could be a crack in the toylet that allows gas to escape. Or it could be a mouse loose in the walls and ceiling peeing all over and smelling it up that way too. I've seen it man! I had to tear apart the walls, just ripped them down to get to that bad smell. Rip, rip rip! I painted over the urine with Zerolac, a white lacquer that covers over smells. They use it for fire restorations. I hated those mieces to pieces!
the Mad Plumber