Routers that come with some manufacturer version of OpenWrt stock would use a sysupgrade to change over to the official build.
According to the developer notes, the u-boot TFTP mode has not been successfully used. It's a dead end for now.
If the router still boots into OpenWrt, the failsafe mode described would be the only possible way to flash without opening the case. Since there is no "system" LED to watch flashing, you're going to have to figure out the timing of pressing the reset button. I suggest connecting the Ethernet cable to a switch so you have a light indicating the port is up. When the bootloader loads, the port will come up. Then when the bootloader starts OpenWrt, the port will go down for a short time until OpenWrt brings it back up. This is when you should start pressing the reset button repeatedly. Do that for about 15-20 seconds then check if you are in OpenWrt failsafe mode.
Again, do not press the reset button immediately after power on, you have to wait for the bootloader to finish so you don't get bootloader TFTP mode. And don't hold the button down, press and release rapidly.
As far as opening the case, it looks like most outdoor CPEs of this design. The main part of the case is an empty tube that has the top, sides, front and back all made as one piece of plastic so that rain can't get in. The entire works of the unit will slide out the bottom, usually after removing one or two screws that are hidden under a label.