Weird gutter like smell in washroom, increases at night

My washroom was renovated about a year ago from scratch, like new flooring, shower, walls, tiles, everything. Like a few days after us using it, there was this weird smell that was lingering like the smell of the gutter backfiring. We got multiple plumbers to look at it and each plumber had their own conclusion. They are the following:

1. The smell was seeping through the lining of the toilet (He put extra plaster or something like that on it to completely seal it).
2. The smell is coming from the drains (Checked the water level in the shower and toiler drains and no smell was there, no mold, nothing)
3. Removing the toilet to investigate (We did not proceed with this as the tiles would have to be broken fully according to the plumber)
4. There is no vent pipe so the smell is coming back into the washroom (He says some breakage of walls need to happen as the air vent pipe will need to be taken out of an existing pipe out of a ledge we have outside our washroom.)

These are the conclusions that the plumber(s) have come down to.

Right now, I do not want to just blindly start breaking everything and try to investigate, If anyone can share any insights on this or maybe double down on any one of the plumbers opinion or maybe give their own, I would have some well needed clarity as its been one year since this issue.

Notes:
We live in hot climate where houses are made of cement and piping is usually within that cement. The smell increases at night severely. The smell increases during winters, much less during the summers. The washroom we had before this had no smell ever.

Thank you guys in advance