Foul gas from toilet

I am suffering from foul gas problem from the potty.My plumber suggests that
the drain to the main sanitary pipe is connected to a T junction is the root
cause.He suggests breaking up the whole system and replacing with a Y junction.I stay on 19 floor and pressure of gas is causing this problem.Kindly advise if this is it?thanks,
Kumail

The 19th floor?

Is the plumbing on the outside of the building, or inside?
What country is this located in?

Toilets in the USA have water in the bowl and trap that prevents gas from escaping back into the room. If the seal between the toilet and the drain is bad, it can escape there. In a properly installed and working drainage system, sewer gas should not be a problem, regardless of which floor you are on.