First question, why are you using two routers? What is the purpose of router B on your network? Depending on what you are trying to achieve with the second router (and/or the details of your network that we'll get into later), there may be a better solution or another way to achieve your goals.
Are both routers running OpenWrt?
You should be able to reach the NAS from devices on both network A and network B when the NAS is connected to router A, but not the other way around (by default from a networking perspective; the local firewall on the NAS may also be a factor). Do you need to connect your NAS to router B, or is router A sufficient?
hi...
I have two routers to make individal networks on different spaces.. 1 to home 2 to office and works fine but now i have this problem
Yes two router runinng openwrt
the porpuse its maintain both DHCP and make discoverable all devices in both routers .. NAS attached on Router A but i want to access to it on Router B (not possible for now) or make NAS discoverable on both routers
A preferable and more efficient configuration would be to have the first router handle both networks -- this makes things more efficient in terms of routing and firewall rules and administration. Your second device would then be a dumb AP/switch.
Alternatively, you can setup a static route on router A for the route to router B's network, and then turn off router B's NAT masquerading and adjust the firewall accordingly on router B. This requires changes to both routers.
I would use VLANS to separate a port on each router, and use it for the wired connection. Create a network interface on that port, with static IP addresses on each end, and DHCP disabled, then add static routes.
As @pshernan said, in your initial connection, it is possible to reach network A from network B. When a client of B makes a request for any IP address not in B's LAN, router B by default sends it upstream to network A.
But when a NAS, printer, etc. is on a different network you will need to know the NAS, printer, etc. IP address and manually enter it into the client PC. Automatic discovery doesn't work across networks.