Postcode Issue

I'm having an issue with the app on the phone, It think its because our postcode field is test and the when i check what the standard are, its set to "numeric" because of zip codes.

What i am trying to get working is when i have a prospect customer, there is a feature that enables you to see how many other prospect customers are around your locations within a miles radius.

in our crm we have our post code set to text and should we require a formula to enable this to work, is this a feature that is only working for zip-codes? or has anyone in the uk got the app feature to work?

Please let me know if anyone can help me, i hope its just a quick fix where i can just export all the postcode data in to a new field that enables the app to work.