I think I have the same question/problem as you. I can use the power remote to power my HTPC off or put it in standby, and if I power it on right away from the power remote, the wol magic packet is sent and the PC wakes up, but once its been off for a while, the app no longer can access the remotes from the sleeping server and there seems to be no way to use WOL other than to install a separate WOL app.
I came from Hippo Remote (which was a great app until Apple killed it with iOS 11 no longer supporting 32bit apps). It always sent a WOL magic packet when it attempted to connect to a server, so the server always woke up right when you started the app because it would attempt to connect to the default server you had configured. If Unified Remote has this feature, I can't seem to find it.
Another feature hippo remote had that I really miss is shake to exit. From any remote, if you shook the phone, it would present a confirmation dialog to exit the current application. I usually like to quit kodi before starting netflix and vice versa (for example), and that feature was really great.
I'm still new to Unified Remote so I'm still learning what it can do. Maybe it can do both of these things and I just haven't found the features yet.