Try making it use a broadcast packet. The etherwake command will use unicast by default, which will probably not survive a hop across a switch port if the target device is in a low power state. I'm not sure about the LuCI interface, but from the hex dump of the packet in that video, it looks like it was using unicast.
Here's how you would do that on OpenWRT command line:
etherwake -i br-lan -b 00:30:18:01:73:fb
Edit: It looks like the LuCI interface has a checkbox at the bottom for using broadcast that is not checked by default. You can also just try it with that checked.
This is the screen shown in that YouTube video you linked. If both the OpenWRT AP and the PC you're trying to wake are on LAN, this should work without issue.