It doesn’t change the distance from the border to the edge of the page, it reduces the distance between the margin and the border.
Did you change the font size for Arabic in the Complex tab of the Font page in Frame Contents style?
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You still have Indent in Default Paragraph Style which means the indent is applied to Frame Contents. You should really have the page number styled as Footer which also has the indent applied so you can’t have it centred. Remove the indent from Default Paragraph, right click it, select Edit Style, choose the Indents and Spacing tab and set indents to 0. Apply the 0.5 cm indent to Body Text.
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You might like to lock the Frame position and possibly Size for the page number