Tata Docomo — postpaid mobile

I have used a tata docomo postpaid mobile number ([protected]) which is under the plan gsm elegant plus 299 with total sum of 353 rs monthly charges.
On 5th september 2017 i have decided to port the network to jio which was activated on 23rd september 2017.
Previously for the period of 05/07/17 to 04/08/17 i just paid 353 rs as one month bill. But for the period after i opt to port that is from 05/09/17 to 22/09/17 they generated the bill amount rs.538 due.
Without any intimation to me they charged as this much for 17 days and there is no proper explanation given to me about the change of plan.
Now please file a complaint against them for charging this amount and not explaining or intimating about the post paid plan.
Now they are even sending me a legal notice for recovery of the bill amount.
Please guide me in this and i have all the proofs about their billings.