The only thing I did was switching it on in OMV's network settings and it works. Btw, my main installation is OMV 2.x with backports kernel but I have a second system with OMV 3.x. Both 'normally' work.
Sometimes my 2.x system did not wake up via WOL and I had to switch it on using the power button. Sometimes that just did the trick and sometimes I had to plug in a monitor and keyboard for debugging. Then it worked immediately though. I have the strange feeling that plugging in a USB keyboard just does the trick. But don't get me wrong, that's kind of 5 out of 100 boots where it didn't work.