I checked immediately after you made your post on 19 June and mine hadn't yet updated at that point.
Now though, my firmware has also updated to 10.87, so I can also now see the proper DHCP Static IP Lease page. A whole lot easier than entering gobledeygook into the URL (which did work, and thanks again Vigi for your efforts in finding the workaround).
It was updated silently by Optus.
As much as Optus should be embarassed over this missing page, I would have hoped that they could have "pushed" a message to end users explaining new functionality with the firmware.
Maybe try a reboot to see if pulls down the new firmware verson.
BTW the Buildtime stamp in the device info page shows 190507_1738.