Tends to be a little messy
First lets assume router 2 has the address of 192.168.16.253 for its connection to router 1 on the wan port.
On router 2 you must do port mapping/forwarding just like you do on a internet router to make the NAS visible to the upper network.
What you now do is on your server you access the nas using 192.168.16.253 which router 2 port forwards.
Now this only works in the most simple cases. You start running multiple NAS devices or things that use dynamic ports and you will need to get a true router rather than these things they call routers that actually gateways.