If the REST API doesn't work, or saving/loading settings in the plugin settings area doesn't work, we should display an admin notice with the message we got from the REST fetch request (if there is one) and a message to check the settings of any security plugin or security setting in their website that disables REST API for administrators.

Steps to replicate:

1. install https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/ and enable Intranet option
2. Go to Stackable settings
3. See that all settings are not loading, you also cannot change settings