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Check the aspect ratio button on your Television remote controller is set to either Auto or 16.9 on the HDMI source that the box is connected on your television.
Sounds to me it's currently set to 4.3.
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Nope, it's not this. Remote does not remove side bars.
I am assuming you are seeing side vertical black bars on every movie with your Cinema Membership and not occurring on one particular movie ie Zack Synder Justice League which has a 4:3 aspect ratio intended by the Director ?