How to sell demat shares?

Hi,

I have some shares in a demat account with a bank for the last 10 years. I now wish to sell them. When I enquired at the bank, the man at the counter said I need to open a trading account with the stock exchange. The bank itself does not facilitate this but he gave me the contact number of a person (a broker) who will assist in opening the trading account. He will charge an annual fee for maintenance of the trading account. I also came across a few apps on Google Play Store (such as Groww) which claim to help trade in stocks/shares with zero brokerage.

My query is this:  I am not interested in trading in shares regularly. I just want to sell off my shares and close the demat account. So is it mandatory to open a trading account? If yes, then which is a safer option....using a broker/brokerage firm or the numerous online apps? Or is it possible to sell the shares online without any assistance from a broker or online apps?

Thanks.