A domain was pointed to AWS S3 Bucket, Cloudfront and registered on Certificate Manager for SSL Certificate, we upgraded the website to WordPress, deleted the S3 Bucket, and then pointed the domain to the Instance IP Address, but browsers label the domain as unsafe (unsecure), please can someone point me to the right direction on solving this problem.
If you are pointing the domain directly to the EC2 instance, then the SSL certificate you created in Certificate Manager isn't being served anymore. You either need to keep using CloudFront and point it at the EC2 instance, or add a load balancer in front of the EC2 instance, point the domain to the load balancer, and attach the SSL cert to the load balancer.
Pointing CloudFront to the EC2 instance is the best option because you really need to have a CDN in front of WordPress.