This function always worked for me until the latest firmware update (I have an iPhone 7). Here are the steps I took to fix the issue:
1. Remove the app.
2. Remove the pairing.
3. Reset the UE Boom (hold ‘down volume’ and ‘power button’ until speakers makes a noise, it will then turn off).
4. Download and install UE Boom app.
5. Turn on speaker (when rebooted it will be in pairing mode).
6. Pair phone to device.
7. Test remote power on/off.
Please give feedback.