Hi there,
Have you installed any new plugins recently? I might suggest disabling all your plugins and seeing if that fixes the issue; if it does, turn them back on one by one to see if you can’t discover which one might be causing the issue.
It’s also possible that a theme change or new theme may be causing the issue. I would suggest that you use one of the default (Twenty*) themes.
Eventually, if you can install plugins, install and activate “Health Check”: https://wordpress.org/plugins/health-check/
It will add some additional features under the menu item under Tools > Site Health.
On its troubleshooting tab, you can Enable Troubleshooting Mode. This will disable all plugins, switch to a standard WordPress theme (if available), allow you to turn your plugins on and off and switch between themes, without affecting normal visitors to your site. This allows you to test for various compatibility issues.
There’s a more detailed description about how to use the Health Check plugin and its Troubleshooting Mode at https://make.wordpress.org/support/handbook/appendix/troubleshooting-using-the-health-check/`
I hope that helps! Good luck!
Beyond performing the previously mentioned WordPress troubleshooting I would suggest to you to reach out to the hosting provider for help in troubleshooting the 504 server-side. They should be able to help identify the cause of the 504.
Good luck!
