Is it possible to run Artifactory on-prem server across multiple AWS regions with a single accessible url?
To achieve your goal you should think about Artifactory SaaS solution.
Otherwise yes, you can have a multi-region solution with several Artifactory instances communicating with each other via federation, but it isn't the same thing.
Here's some documentation about it:
https://jfrog.com/blog/our-solution-for-scalable-multi-region-saas-deployment/
https://jfrog.com/knowledge-base/how-to-configure-federated-repositories/