networking – How to access LAN of another router connected via WAN port?

I have 2 routers connected via WAN-LAN. Base router that have internet connectivity and another is used to extend network. Both networks are working fine. For some reasons I need to access few devices that are connected to Router 1 with a computer connected to Router 2. Image showing sample connection diagram

As per the image I want to access Data Server via Computers directly, like I share drive/folder over LAN.