Cannot load anything other than root request -- ECS + ECR + ALB 503s

Greetings,

I have a simple static site behind NGINX in a container. I have pushed the image to ECR, and can pull and run the container without any issues.

However, when I put an Application Load Balancer in front of the ECS Service with Fargate, I am unable to load any assets. For that matter, anything beyond the root does not load: i.e. subdomain.example.com loads, and subdomain.example.com/* throws a 503.

As mentioned, I can run the container locally without issue by pulling the :latest tag, so I am confident it's not an issue with the container, and the issue lies somewhere in the configuration of the ALB and ECS Service.

Any guidance would be greatly appreciated.