After a lot of research following setup worked:
Connect Optic Fiber cable to FTTH Modem, reset FTTH Modem to factory setting OR delete all configuration parameters so that it's configuration matches with factory setting, now configure it in bridge mode only. Please don't put user id & passwords etc.
Connect any common Router (Wi-Fi can also be used depending on need), with FTTH Modem. Please ensure that IP Address series should be same for both devices. Generally FTTH Modems comes with IP Address 192.168.100.1 and Routers have 192.168.1.1. Please don't use ADSL Modems / Routers.
Configure router in bridge mode with dialer values i.e. login id, password, static ip, gateway etc. in Router
Configure port forwarding / virtual server as per the need.
Everything configured, now enjoy fiber optic high speed flawless internet service.
Cheers!!!