Our plumber has put a tundish (not a dry trap one) next to our unvented cylinder, and it is connected to the waste pipe and we get a smell from the stack (there being no trap).  Our plumber says that the tundish cannnot be replaced by a dry trap one designed for unvented water tanks and boilers (Hotun Hilfi).   There are two tundishes and it is not the one close to the T&P relief Valve on the cylinder.  

 

Is there a reason why he cannot put a dry trap tundish ? 

You still have the tundish, followed by a waterless trap.