Hi I recently bought a avenger 220 cruise abs model. It's been a month

Hi I recently bought a avenger 220 cruise abs model. It's been a month and the bike is vibrating when I am I at 3rd gear and speed of 29km/hr. I told the service people the same and they are like need to change gear to 4. I can't even got to speed 30 at gear. Is it okay for Avenger. I have very less knowledge about bikes. Anything would helpful.

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