furnance vent flue proper attachment

I had a code inspector let me know that my HVAC furnace flue is not properly attached. I have looked all over the internet and i think its a new code requirement in my area in the past few decades (the house is from the 80s), so I need to get the attachment updated.

online I see a few examples of using heat-resistance silicone, or sheet metal with screws, to attach the flue.

my flue is simply fitted onto the exhaust.

does anyone know what is the proper fix for this? if it seems hard enough i'll probably call an hvac service to come do it, but I'm not even sure what to ask for.

EDIT: I was able to get a hold of a Proflex 2 in. Neoprene Shielded Coupling, but it seems like its the wrong size. the next size up is 3 inch. will that work?