I did that. I went from the latest stock Asuswrt to Fork so had to use the FW Restoration Tool. I restored settings to default, did WPS NVRAM clear, flashed firmware again but through GUI and repeated the above.
It may need to be flashed with the FW Restoration Tool again. Might try it once the RT-AC86U comes back from RMA.
Try using FireFox, with out any extensions, just bare bone/vanilla installation of a FireFox. 99% of the time, this works flawlessly for me on Windows and MAC machines.
For the 1% of the time, I use Asus Firmware Restoration tool.