As you can access the NAS fine via its IP address, the NAS is reachable from your Android device – but the issue is, your Android device cannot resolve its name. That is because Android, by default, always uses Google's name servers, no matter what the DHCP server tells it. You will have to override those settings:
- Open Settings (e.g. from your app drawer) and go to WiFi
- Scroll the list of WiFi APs and long-press the entry of your own WiFi network
- Chose to edit the entry
- Tick the check-mark for "Advanced settings"
At this place, you will probably see it has "DHCP" selected. To edit the settings we need to change, you'll have to switch that to a "fixed address".
- Switch from "DHCP" to "fixed"
- Check the IP address and make sure it matches your network1
- Make sure the "default gateway" matches the IP address of your router
- Change the value of "DNS1" (usually
184.108.40.206) to your name server (usually the same as your router), but leave "DNS2" (usually
- Save the settings
Now try again: you should be able to access your NAS via its name, the same way you can do from your PC. As we left "DNS2" untouched, names your router cannot resolve will still be resolved via this. Once you have verified it works, you can try switching back to DHCP and check if it still keeps the changed name server, and thus leave the task of managing available IP addresses completely to your router (avoiding address collisions in the future). If switching back to DHCP brings your original issue back, you'll have to stick to "fixed IP".
1: you can find the original IP address in Settings › About phone in the status section