This is a simple thing that you can change the settings to use your FTP on Airtel  Router modem as it needs to make enable connection.

Enabling the FTP ALG (Application Layer Gateway) in your router modem will allow it to properly handle FTP traffic. Here’s how to do it:

1. Log in to your router: You’ll need to access the router’s web interface to change the settings. To do this, open a web browser and enter the IP address of your router into the address bar. You can find the IP address in the manual that came with the router or by using a network scanning tool.
2. Navigate to Advance > NAT > ALG: Once you are logged in to the router, navigate to the ALG settings page. The exact location may vary depending on the model of your router, but it should be under the “Advance” or “NAT” section of the settings.
3. Enable the FTP ALG: On the ALG settings page, look for the FTP option and enable it. Save the changes and exit the settings page.
4. Test the connection: After enabling the FTP ALG, test the connection to ensure that FTP traffic is flowing correctly. You can use an FTP client like FileZilla to connect to an FTP server and see if the connection is working properly.

Note: The steps to enable the FTP ALG may vary depending on the model of your router. If you are having trouble finding the correct settings, consult the manual or contact the manufacturer for assistance.