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If you are using the Arcadyan gateway you will find you need the mobile application to better manage the gateway. Once your phone connects to the local WIFI then you can get access to the available management options. The Arcadyan gateway is rather limited compared to the prior Nokia gateway which still allows management via the web browser interface. 

In the app, click “Network”  you can add another network by pressing the magenta + symbol, give the new network a unique name, change the  frequency  to 2.4 only and the protocol to WPA2, leave the rest as is.  Let the router update then search on your device for the new 2.4 network.

I suspect you have the Arcadian GW and it has updated to the newer code that doesn’t have the web management access. From the community conversations I have seen on the subject I am pretty sure you will need to use the mobile application for management of the gateway. I have the prior Nokia gateway and it does support the browser management so I cannot say for sure. 
There was a firmware update to that gateway which resulted in that change. It is not a good change for the end user experience but I don’t think that was the focus of that decision. Management of the appliance without the option for a physical link is a poor solution and makes it less flexible.