Ok, first thing's first - what exactly is happening when you try to use On Demand? The more specific you can be, the better we're able to advise.
Let's also check that your box has a working internet connection, as that can often be an underlying cause. Follow Home > Help & Settings > Settings > Network. In the top-right, does the box report connected? There's a few other bits that can be attempted depending on the results of this.
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