Plugin not working properly

We are having several issues with the plugin but the most significant one is that someone managed to purchase a product in USD when the only currency we have set up as the checkout currency is GBP…big issue, as the vendor lost 20% of the sale.
Another issue is that it doesn’t always recognise the country where the user is from. Users logged in the UK will see the pages in USD for example…in other tests, on one page you’re in USD then you click on a product and it appears in some other random currency…
Shortcode for displaying currency exchange rates is not working either.
We need assistance to resolve this as this is not sustainable.