OAuth Times Out

When I try to set up the plugin initially, I follow the guide here: https://bjorntech.com/kb/getting-started-with-the-woo-zettle-integration/

When I click ‘Authorize’, I get a connection timed out message that looks like it is on Zettle’s end (or maybe the plugin, I am not sure).

Here’s a link to a screenshot of the error as it appears in Firefox.

I can’t progress any further and I am not sure what to do.

Thanks for your help!