If may have need two wax rings if the flange was too low.
One wax ring only works if the flange is above the finished floor.
If you have a lav in the bathroom, that should be vented, even if it's an AAVthis bathroom was just stubbed out, without any fixtures.
Then you will need at least one vent going through the roof, and extending at least six inches above it.
There is no excuse for the builder putting indoor plumbing in a structure without properly disposing of sewer gas from a septic tank. Was he just lazy and cheap?