Bluetooth headphones show as 'connected', but the audio doesn't work

I'm having this problem with my Bluetooth headphones, a Tronsmart Apollo Q10 to be exact.

Basically I'm trying to connect to these headphones in Kubuntu (the latest version as of today), they appear on the pc, and I'm even able to click the connect button. When I do so, it also shows as 'connected' on my devices list, but there is no option to use it as output for audio.

I have the suspicion, however, that it doesn't really connect to the headphones, as the headphones play a voice saying 'audio connected' when connected to any of my Android and Windows devices, but it doesn't do so here.

I've tried multiple solutions already, like installing blueman or ofono/ofono-phonesim, but I've had no luck.

Here's the list of my Bluetooth devices if that's of any help:

lsusb
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
http://www.Hexgears.com258a:002a SINO WEALTH Hexgears keyboard
Bus 003 Device 004: ID 256c:006d  
Bus 003 Device 003: ID 08bb:2902 Texas Instruments PCM2902 Audio Codec
Bus 003 Device 002: ID 046d:c539 Logitech, Inc. USB Receiver
Bus 003 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 003: ID 0a12:0001 Cambridge Silicon Radio, Ltd Bluetooth Dongle (HCI mode)
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Thanks in advance!!

Maybe it's too late, but I had the same issue, and the solution I found was in https://unix.stackexchange.com/questions/258074/error-when-trying-to-connect-to-bluetooth-speaker-org-bluez-error-failed/258076#258076.

This is the process I followed:

1. Turn the headphones on in pairing mode (pressing the power button for a long time, not only until they are on).

2. Discover the address of the headphones. It must something like XX:XX:XX:XX:XX:XX. You can see it in Bluetooth Configuration > Devices > Tronsmart Apollo Q10 > Address. Copy it.

3. Install bluetoothctl, with sudo apt install bluetoothctl

4. Now type these commands on a terminal:

   $ sudo bluetoothctl

   [bluetooth]# power on

   [bluetooth]# agent on

   [bluetooth]# default-agent

   [bluetooth]# trust XX:XX:XX:XX:XX:XX

   (with XX:XX:XX:XX:XX:XX being the mentioned address)

   [bluetooth]# pair XX:XX:XX:XX:XX:XX

   Wait until you can read "Pairing successful"

   [bluetooth]# connect XX:XX:XX:XX:XX:XX

   Wait until you can read "Connection successful"

   [bluetooth]# quit

   Once done, everything will work correctly. This is not something you have to do everytime you want to connect the headphones: from this time on you can just connect them as you did before, but now it'll work.

   Hope this was useful!