First of all, your car purchase will be covered by the Consumer Rights Act 2015. This requires the car to be of satisfactory quality, fit for purpose and as described. A radio that won't connect to your smartphone won't cause the car to fail the 'satisfactory quality' or 'fit for purpose' requirements, nor could you argue the car wasn't as described - unless the dealer explicitly said in the advert that it'd be able to connect to your phone.

So that leaves the warranty. The dealer probably uses a warranty from a third-party provider and these vary in quality quite significantly. It's unlikely that it'll cover your issue, but check the terms and conditions of the warranty you should have been given when you bought the car. If you think you have a case, take it up with the warranty company rather than the dealer.

Finally, while it's frustrating, the dealer probably has a point that it's a big ask for an 11-year-old car to work seamlessly with a modern smartphone. The best idea is to suck up the bill from your local Porsche Centre.