Critical Error message when loading plugin

When loading this plugin from the admin, I get the following error:

There has been a critical error on this website.

I turned on debug in wp-config, and the page then threw this error:

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, non-static method QuadLayers\WCDC\Controller\Premium::add_page() cannot be called statically in /home/790xxxx.cloudwaysapps.com/dsfasfxp/public_html/wp-includes/class-wp-hook.php:308 Stack trace: #0 /home/790xxxx.cloudwaysapps.com/dsfasfxp/public_html/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters() #1 /home/790xxxx.cloudwaysapps.com/dsfasfxp/public_html/wp-includes/plugin.php(517): WP_Hook->do_action() #2 /home/790xxxx.cloudwaysapps.com/dsfasfxp/public_html/wp-admin/admin.php(259): do_action() #3 {main} thrown in /home/790xxxx.cloudwaysapps.com/dsfasfxp/public_html/wp-includes/class-wp-hook.php on line 308

I’m not quite sure what to do. Can someone perhaps point me in the right direction?

Thank you so much!

The page I need help with: [log in to see the link]