Ansmann is a German battery brand that have started selling a lot of budget RC stuff in the UK.
If the car is brand new and has the supplied kit motor there is no way it should only last 5 minutes because that is not a very powerful motor. Really you should have at least 10-15 minutes runtime.
Are your 4200 batteries new or second hand? If they are second hand they may already be knackered. If they are new, make sure they are getting fully charged, they should be warm to the touch when they peak charge, sometimes you can get false peaks so if the battery is cold, start the charge again.
Another thing to check is that the car rolls freely when the motor is removed, if something is binding that will make the car slow, overheat, and run out of battery power quicker.