Thank you for reaching out!
In most devices, WiFi AP functionality is only used to connect the device to a WiFi network, and after the setup, the device becomes a WiFi Client. So your device does indeed seem unusual. In this setup, the device will not receive an internet connection. It would have to be configured as a WiFi client to achieve this.
As I understand, you'd like to reach that device (acting as a WiFi AP) via your WiFi network, provided by RUT240, correct?
If yes, then this should indeed be possible. The most important factor is for both of the devices to have different LAN networks. For example, if your end device has a network of 192.168.1.0/24, then RUT240 LAN IP should be changed to something different (e.g. 192.168.5.0/24).
Then navigate to Network → Wireless and perform a scan of the neighboring WiFi networks. Your device AP should show up here. Enter the correct password and assign it to the LAN network.
After that is done, navigate to Network → Interfaces → General, and make sure that the created network is at the bottom of the list. This way RUT240 will not try to use it as an internet source.
And that's it! Now, after connecting to the RUT240 WiFi network, you should be able to enter your end device's gateway IP address (the same IP, that was used to access the device when connecting directly) and access it's interface!
Let me know if you encounter any issues!