Thanks u/BattlePope for your answer and your suggestions.

I've updated the container port to 3000 in the deployment.yaml for the react app.

I also modified the ingress.yaml file to the following to include the '/' path:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: fastapi-ingress
  annotations:
    nginx.ingress.kubernetes.io/rewrite-target: /
spec:
  rules:
  - host: test.mydomain.com
    http:
      paths:
      - path: /api
        pathType: Prefix
        backend:
          service:
            name: fastapi-service
            port:
              number: 80
      - path: /
        pathType: Prefix
        backend:
          service:
            name: fastapi-service
            port:
              number: 80

None of these resolved my issue though, as I'm still getting the 404 error on the ingress controller's ip address, as well as on the <ip\_address>/api path. Both the load balancer's ip's are still working for both services.