You have made the mistake which majority of the citizens make.
The citizen/customer should issue orders to disconnect in writing under proper acknowledgment and obtain complaint/Transaction/unique number……………..from office/customer care.
If you have the customer care call detail: date, time then mention that despite your lodging the compliant there was no redressal of grievance and speed was not restored and despite your order to disconnect the connection is not disconnected.
Let the reply to the legal notice be by your lawyer and let your lawyer affirm that jurisdiction is your location and not Delhi.
TRAI has issued detailed guidelines for the protection of consumers and same are available at its website e.g;
TRAI Consumer Protection Guidelines, FAQ, Broadband Quality of service regulations
The decisions given by Dist, State, and National Consumer Disputes Rederessal Commission for this and all companies can be downloaded from:
Confonet
confonet.nic.in/