Although Bluetooth generations should be backwards compatible, I note the S11-HDs are BT3, whereas the Ionic is BT4. I know the Ionic will pair with non-Fitbit headphones, as I’ve paired my B&O E8s. However they are BT4.2.
Short of trying again and making sure both your headphones and Ionic are in pairing mode (for the Ionic: Settings>Bluetooth>+New Device). Also, make sure your headphones aren’t already paired/connected with something else, like your phone if previously paired. That is all I can offer, sorry.
