Unable to use LB1120 interface after setting to gateway mode

Once I set the LB1120 to gateway mode the web interface is no longer available. The IP address is assigned by the ISP and 192.168.5.1 is nonexistent. I have to factory reset to put it back in router mode.

It was claimed this issue was fixed in a firmware upgrade in 2017. This has been a continuing problem and none of the solutions I have found here have resolved the issue.