I work on trucks wear the hole braking and fuel system and air system is made up of plastic pipe like that going from 4mm too 12mm and the hot water trick works fine also if your very very carful you can soften it a little with a heat gun. Ive done both of these many times on the side of the road lol and it's been fine!
But really you need a tool like this to clamp round the pipe then knock the fitting in