Dust didn't do that, a cracked solder joint in the horizontal deflection
area did. The obvious clue is the lack of deflection when it acted up. 5
minutes with a soldering iron *before* the HOT and possibly other components
blew would have saved your monitor

I've had two Iiyama monitors with that fault. Just needed to get
underneath the main circuit board and resolder the connectors to the
scan coils.