here's my situation:
UE BOOM 2 speaker ver. 2.1.107
APP version 6.1.96
iPhone6S OS 11.2.1
I had an old BOOM1 and the same phone, remote power worked just fine. I smashed the BOOM1 by accident.
Bought a new BOOM2 and brought it home, remote power didn't work at all. Same phone, the app was probably updated in the meantime. Started a ticket with support, they were useless and ignored me.
SO. I read all this and decided to try this method and it worked for me. Just like it says above, reset the speaker by holding down the " - " button and the power button till you hear a sound, then delete the BOOM app and re-install it. Turn on bluetooth and pair the BOOM 2.
Remote power works fine for me now. Weird. Hope this helps.