In Magento 2.1.8 website . currency rates does not automatically(cron) not working.
You can also show my "Scheduled Import Settings" into Below image
Magento Stack Exchange is a question and answer site for users of the Magento e-Commerce platform. It only takes a minute to sign up.Sign up to join this community
After change the setting run:
php bin/magento cache:flush
For automatic rate updation to work, you need to make sure that Cron settings on your server.
For more detail check : Link