Websites are already accessible through IP as default. If you want to specify which one of multiple website is served without domain-specific request, add a Port 80 entry to it with the Default option checked.

What is the error exactly?

THIS. This is SELinux denying connection to NGINX.

Have you tried restarting NGINX?

Could you post the domain name?
Try to tail -f all available logs in /var/log/nginx while accessing the website, in case NGINX is logging into a wrong file.

Are you 100% sure that you're actually accessing your server when seeing the 502? No caching proxies, nothing?

That's impossible - check that error_log path is set in your /etc/nginx/conf.d/website.conf. If it's there, and NGINX config test succeeds, and you see an error page - error info should be in the log.

Could it be that Ajenti has misconfigured your NGINX? Check with nginx -t.
If you see a 502, there has to be an error message in the logs. Either in /v/l/n/website.error.log, or in /v/l/n/error.log

Also, make sure that check /var/log/nginx/website.error.log for more info.