They were paid to install a map, which theyve done.
Er hmm..
Fella.. You PAID them to map your car not simply "install" it, not set it up and not check its function is as it should be laving you without the stage 2 level it should be. N249 faultcode is not the cause..
If you are being fobbed off thereafter your first recourse is back with them. They can do more than they have done for you currently.
A vag specialist will likely SAVE you money at this point, cutting to the chase and diagnosing the issue(s) quickly, certainly quicker than the poke and hope approach which you have at your disposal, without the correct diagnostic tools, and spares, and importantly Experience of the 1.8t
Find a local vag specialist, ideally who is also a revo dealer and one with a dyno, smoke tester etc so meaningfull diagnosis can be made
There are limits to whats possible advise wise without data or seeing the actual car
Your "tuner" at face value from how it reads here has done minimal work for you beyond simply flashing software into the car, and any monkey with a flash tool & laptop can do that, and it does'nt make them a "tuner" by any means.
See if they can log your car, vcds on the dyno, do smoke test, IF and only IF you believe they have suitable VAG experience.. IF they're Revo dealers and not vag experienced I would be most worried about their suitability for being a Revo dealer who's majority base is VAG cars.
Good luck