@sagar002100 

Hi, suggest you check the configuration FAQ first to learn some details.

If you need to put the ER605 in the Branch Office network, suggest the connection is GPON router--- ER605----Clients.

And here will be the double NAT in this network, so you need to open the port for R605's WAN IP on the GPON router.
And if you use the IPsec VPN, need to open UDP 500, UDP 4500.

Then when the VPN client try to connect to the VPN server, you need to type in the GPON router's WAN IP (not R605's) on the VPN client router.

  @sagar002100 

You may have some additional options.  Quickest is to make the ER605's IP the DMZ IP on the GPON modem, that effectively makes all public IP ports arriving at the GPON modem available as if they arrived on the WAN port of the ER605.

You may also be able to establish a connection within a connection by configuring your PPPoE account settings currently configured on the GPON modem on a WAN port of the ER605 and then connecting that WAN port to a LAN port on the GPON modem.  This may enable you to get a static IP via PPPoE directly on the ER605.