Thank you for your reply, I'm just writing an update in case anyone had a similar problem.
I hired a professional and he found multiple sources for the odor problem. The drainage pipe was clogged but it wasn't the main culprit. The smell was coming from the crawl space because the return duct that passes through it was damaged and it was taking air from the crawl space rather than from the inside of the house only. I was told I should consider installing an insulator on the bottom of the floor separating the crawl space from the living area because it was contributing to the odor. Sadly, for me, that isn't an option considering it's very costly and doesn't guarantee to solve the odor problem. For now, the drainage pipe is fixed and I will seal the return ducts soon and hope that would be adequate.
Thanks again.