Not helping with DOS attack

A site that I manage has had an uptick in login attempts without a username in an attempt to bring the site down. So, I installed this plugin to prevent the robots from being able to submit in the thousands like they’ve been doing.
Strangely, I see no difference in the attack numbers, even though this plugin is installed and working.

Any ideas if I somehow did something wrong, or what I can change to add more security? I did raise the number for how long it takes to calculate in the browser.

Thanks.
Dan

https://www.ads-software.com/plugins/hashcash/