Hi,
The IoT network created on the main Flint 2 is a separate network from the Main LAN. When the second Flint 2 is connected in AP mode through a normal Ethernet connection, it will not automatically inherit and extend the IoT network configured on the main router.
If you want the second Flint 2 to provide the same Main / Guest / IoT networks while keeping them separated, VLANs are required to distinguish these networks over the Ethernet connection between the two devices. The connection between the two routers needs to carry the corresponding VLANs, and the VLANs can then be mapped to the appropriate SSIDs on the second Flint 2.
If this is what you would like to achieve, you can refer to the following example from another model/firmware version with similar requirements:
Thank you for your support and understanding.