The geolocation logic used by the Tax Display by Country plugin is the same used by our flagship product, the Currency Switcher for WooCommerce. Please refer to following the knowledge base article for a list of the most common issues related to geolocation, together with their solutions: Currency Switcher for WooCommerce - Geolocation doesn't seem to work properly - Causes and solutions.
Although the article mentions the Currency Switcher, the solutions are applicable to the Tax Display by Country plugin as well.
You can purchase the Tax Display by Country plugin from our online shop.