backfire is when the combusted gasses in the cylinder dont have any access to the exhaust and so they shoot back through the intake manifold or the headers. in urdu, it is usually called a "patakha"
yes the problem is quite common with corolla's.
first of all, get your air intake box repaired or replaced if it has been damaged. then get the spark plugs and the airfilter changed. Also, take your car to a mechanic and ask him to check the coils (the car will usually run fine on petrol since petrol only requires the coils to be working at 70-80% efficiency, but cng requires the coils to be 100% efficient). If all of this doesnt fix the problem, then replace the cng advancer with one from landirenzo. all of the above steps will solve the problem by about 90%. the final step you have to take is that if the car engine is cold (ie it's the first time you are starting it after 4-5 hours), then run the car on petrol for about 5-10km's before you switch to cng.
hope this helps.