I'm not sure, but it sounds like you're trying to boot on the third time instead of the fourth, so just to make sure we're on the same page, the process to switch to the other partition should look like this -
turn on/turn off
turn on/turn off
turn on/turn off
turn on/leave on and let boot....
If that doesn't work and you want to try reflashing over ssh, you can use sysupgrade, if you want to read up on it you can do so here - https://wiki.openwrt.org/doc/howto/generic.sysupgrade
But for now here's the simple instructions, if you're flashing from OpenWrt you need the sysupgrade.tar file (the .bin file is for flashing from Linksys firmware), so copy the openwrt-kirkwood-linksys-audi-squashfs-sysupgrade.tar to the /tmp file of your EA3500 and then ssh into your EA3500 and run the sysupgrade command -
sysupgrade -n /tmp/openwrt-kirkwood-linksys-audi-squashfs-sysupgrade.tar
The -n option is to erase previous config files so you'll have a nice clean flash to work from, hopefully that will get you up and running.