We assume from your comments, RoyL, that your machine has fuel injection and a O2 sensor? The ones for Europe and USA were set up like that, whereas bikes for the Indian market, and perhaps Indonesia had EFI, but with a "look-up" table type of ECU computer and no 02 sensor. The ECU can be reprogrammed anyone with the Royal Enfield software, (dealers), and there are aftermarket controllers available. Also, it isn't uncommon for people to replace the entire EFI system with a carburetor. The 350s for India only still have, and manage to meet BS IV emmissions standards with, carburetors.
It's never hurts to say exactly what equipment you have on your bike, when asking for advice.
Since your bike quite possibly came from the factory with the issue you described, it may be particularly difficult to diagnose. It's unfortunate that your local dealer is no longer in business.
Taking it step by step, and verifying each component will ultimately find the problem. I don't know any way to test the ECU computer, but every sensor can be checked, and REs are quite simple mechanically, as motorcycles go.
You may want to consider investing in a carburetor, simply as a diagnostic tool, even if you prefer EFI. One nice thing about these Indian bikes, all the parts are quite cheap, via Ebay or Amazon, from suppliers in India.