I looked up the manual and there is nothing regarding rear connections, even in the specs!!
Then I looked for images of the rear panel connections and found another model review, which may be similar: https://au.pcmag.com/tvs/49240/hisense-55h9d
After you make the connection you usually have to turn ARC on in the TV menu. You may also have to turn the TV speakers off and change the audio output type.
Make sure to select the HDMI-ARC as the input on the soundbar.