@poH_ 

From what I understand, the IoT network is not isolated from the main network and devices from both networks can communicate with each other. The IoT network is simply another SSID for wireless connections.  Devices can be isolated by placing them on the isolated devices list (under Advanced, Security, IOT_SECURITY).  Listed devices can communicate with each other but not with un-isolated devices.  For example, if you isolate all your IoT devices that connect to the router using the IoT network, then those devices will not be able to communicate with devices on the main network (and vice versa).

Note that if one of your routers is in the access point mode, then device isolation is not available for those devices that connect to the access point.  This is true even if the devices are listed on the isolated devices list of the main router.

One last note:  The IoT network is not part of any EasyMesh network that you may have configured.  Its SSID is not propagated like the main network's SSID through any range extenders or satellite routers.