My RT-AC66U_B1 reported a firmware upgrade this morning. As before, going to Administration - Firmware Upgrade gave me the upgrade progress bar with no upgrade at the end. Using the Android app also failed the upgrade. I downloaded the 384-20308 upgrade file, rebooted the router, went to the Administration - Restore/Save/Upload Settings tab then back to the Firmware Upgrade tab, clicked the Upload link, found the file and the firmware upgrade ran successfully! I had also cleared the Firefox cache and offline data. Am using Firefox ESR 52.6.0 64 bit.
As in all things computer, your mileage may vary! Best of luck!!!
As in all things computer, your mileage may vary! Best of luck!!!