@iVaibhav Why would you want to "unconfig" the original router? Set it off to the side so you can swap it back in if need be, until everything is working and stable. Replace it w/ the R605, login in, put in your credentials, hit "connect". What happens after that?

@YATechMonkey ,

Thanks for your Response,

So Here is my Problem, I cant set the router off to the side as It has Fiber connected (Acting as PON too)

My Current Setup is with ZTE F602 which has Fiber Cable Connected to it and has a LAN Cable going to a Simple Switch which serves internet to All Rooms

Also it serves as Wi-Fi 

Now , What I did was 

1. Removed LAN Cable from Switch and Connected to R605 WAN Port 

2. Connected a Cable from LAN Port of R605 to Switch.

Accessed the E605 Page from Desktop , This part works fine. 

When I go to WAN Page, and select Dynamic IP , The right hand side shows Disconnected.

Now, Should I select PPPoE and enter the ISP Credentials here ?? and if I do, Do I need to remove it from ZTE F602 , or it shoud work with Dynamic IP as ZTE is already authenticating from ISP and getting an IP from them.  

This is where I am stuck.