I have Raspberry Pi (with osmc), turned on Tethering from WiFi to Ethernet. Raspberry is a WiFi client on my home WiFi network with IP 10.0.0.15 and gateway 10.0.0.138. I have connected MyBookWorld NAS to Raspberry Ethernet, the eth0 has IP 192.168.0.1 and NAS have 192.168.0.2. When I ssh to Raspberry over WiFi, ssh email@example.com, I can ping NAS with ping 192.168.0.2. But I can not ping NAS from my other WiFi clients like Mac with IP 10.0.0.6. Routing to 192.168.0.2 does not work, probably because WiFi subnet is 255.255.255.0. I believe it would be a piece of cake for someone to advise me how to access NAS at 192.168.0.2 from 10.0.0.x clients through Raspberry as router. Thank you very much!
I assume you are using a Raspberry Pi 3B/3B+ with Raspbian.
The problem is not the RasPi. The problem is the home WiFi router with ip address 10.0.0.138. It does not know that it has to send ip packages with ip addresses from 192.168.0.0/24 to the RasPi so that can forward it to the NAS. You have to set a static route on the home WiFi router to the RasPi with destination
192.168.0.0/24 and with the gateway/next hop
If you do not have access to the home WiFi router or if it doesn't support setting static routes then you could use a bridge on the RasPi that would bridge interfaces wlan0 and eth0 so all devices incl. the NAS have all ip addresses from
10.0.0.0/24, means they are all connected to the same subnet. But the problem here is that the on-board chip does not support bridging of a client WiFi connection so we have to use workarounds. But they are not very easy to setup. Keywords here are NAT, port forwarding or proxy arp.
Another solution would be to invest into a USB/WiFi dongle that supports WDS/4addr, so you can bridge the WiFi client connection.
As you see, the best solution is setting a static route on the home WiFi router if possible. Tell me if you are not able to it.