Hi @aristos,
The issue when not running on the same compose is probably because either the network is not reachable or the address is not resolvable. When you apply a dynamic configuration with the File provider you assume Traefik can reach directly to the addresses specified under servers:, in my example you would assume postgres resolved to a network address in the same network as Traefik.
As for Apache Tomcat and HTTP I don't have any ready examples at hand but the structure should be pretty much the same as for postgres, except maybe you want HTTP routers instead of TCP.
If anyone else have examples on this please follow up as it would be really helpful here and also on this other post 