On the Zima Blade the simplest guaranteed reset is to remove the CMOS battery, wait a few seconds, then put it back in. That forces the BIOS to drop all settings and restores the default video output.
The RST-SW header won’t do that, it only reboots the board, so the wrong display mode stayed active.
Pulling and reseating the CMOS battery brings everything back to factory defaults, including HDMI.