adding a gateway in r6900

> from fiber ONT , [...]

   What is it?  Maker?  Model number?

> [...] lan to lan r6900(not using wan) , [...]

   If your "fiber ONT" really os an ONT, and not a NAT router, then why
would you expect that to work?

> [...] i want to use r6900 dhcp and not my fiber ont

   Why?  Is there some actual problem which you are trying to solve?

> [...] or use ONT DHCP which i dont wanna use.

   If your "ONT" has DHCP, then it's more than an ONT.  What is it?

> Untitled.png

   Why the odd-ball LAN IP address?

   Start again.  Describe your equipment and your goals.  Forget about
how you think that you can (mis-)configure the R6900 to do whatever it
is that you actually want.