Airtel — cant able to use internet in postpaid sim

Address:

Chennai, Tamil Nadu

Myself vinoth kumar, my airtel postpaid sim number is [protected].
Currently i am using 199rs plan (80mins, 300mb, 40mins roaming), today only changed the plan into 199rs (I. E 170mins, 200sms, 40mins roaming)
I wont add internet pack because i have unused data of previous month of 8.7gb
I call to customer care, they told u cant use internet because u won't add internet pack, only solution is activate separate pack or change plan to higher plan.
I tried to change the plan in that is showing only once you can change your plan...
But i am not interested to change to go for higher cost plan, i am comfortable with 199rs plan only... Kindly give proper solution, otherwise i ll go for someother network
Myself i want this plan only 199rs (170mins, 200sms, 150mb)

Regards,
Vinoth kumar. S
[protected]
[protected]@gmail.com