The «European Central Bank Currency Converter» Magento 2 extension retrieves currency exchange rates from the European Central Bank as a more reliable alternative to the built-in Magento WebserviceX.NET currency converter, which sometimes does not work (see also the issue 2757).
The extension’s code is on GitHub: https://github.com/mage2pro/currency-convert
How to install
composer require mage2pro/currency-convert:* bin/magento setup:upgrade rm -rf pub/static/* && bin/magento setup:static-content:deploy en_US <additional locales, e.g.: de_DE> rm -rf var/di var/generation generated/code && bin/magento setup:di:compile
If you have some problems while executing these commands, then check the detailed instruction.