UzEE
Occasional Visitor
There's your problem then...
Rescue mode does not do any validation because it's intended for all sort of cases where flashing may otherwise fail (for instance, due to invalid nvram values that would cause any validation process to flash).
If the regular webui fails, then hnd-write will also fail. It does the same model + CRC validation checks as the webui.
The only way to recover is either by reflashing in Rescue Mode, either using Asus' tool, the recovery webui, or a TFTP client. Personally, the only way I always had 100% success recovering from a bad firmware was using a TFTP client. Asus's recovery tool often fails mid-upload for some reason, and the webui does not seem to be present on all models.
This is where I'm stuck right now. The router does not go into Rescue Mode anymore.
It always just boots up normally, and I'm not aware of any other method of putting it into Rescue Mode apart from holding the Reset button and plugging in the power. Could it be that flashing the wrong firmware has somehow remapped the function of buttons / LEDs?
For example, the power LED never turns on now, no matter what. In fact the only LED that lights up is the leftmost one (LAN4 I believe). The LED toggle button on the back also doesn't do anything anymore.
And yes, the regular Web UI now does give a validation error if I try to flash the correct AC86U firmware since it now likely expects AX86U firmware images.