Hi @Blairuachdair ,
The first thing to check is that you see link lights on the NAS port and on the router switch-port so that you know a physical connection is made.
The second thing is to think back to whether you always accessed the NAS via the same IP address, which suggests that the NAS port might be set to a static IP address, or whether your NAS IP address occasionally changed which means it was set to DHCP and got a new address automatically.
You should be able to check how the NAS was mounted (Linux) or mapped (Windows) to find the last IP address it used.
The next thing to do is to check whether the subnet of your new router is the same as for the old router. So you will access the new router by an address such as 192.168.x.x and would have accessed the old router by a similar, same, or slightly different IP address.
Now if the subnet of your new router is the same as for the old router, everything should have worked immediately. After you have performed the above checks, the simplest and easiest way to address can be chosen. (I assume you've already rebooted your NAS a few times and find it nowhere in your router GUI.)
The reason why you see your wifi devices in your router is because they have interfaces set to DHCP. If you NAS port were set to DHCP, you would expect that it would receive a new correct address in the subnet of your new router and you would already have found it in the GUI.
Edit: I thought I would mention a quick way to resolve if you know how to configure your NAS and set its port to DHCP. You can bring up your old router without a SIM inside, connect up your NAS to the old router, and connect your usual configuration device via wifi/cable to the old router. You should then have access via the old familiar IP and method and can set the NAS to DHCP and plug into the new router where it will obtain an appropriate new IP address (a few reboots may be required).
