You need to use a cement board underlayment that is rated for exterior applications. Check the manufacturers specifications for products that are available in your area.
Hardie underlayment is not recommended for exterior applications so if that's whats on there then it needs to be removed.
Each manufacturer of thinsets has products that are rated for exterior use. You need to check whats available in your area and refer to their material data sheets to get actual product details on which thinset and or additive is best for the intended application.
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Randall