i'm no big fan of the toml notation, but shouldn't it be
[http.services.foo.loadBalancer]
instead of
[http.routers.external.foo.loadBalancer]
Maybe this post could also be helpful: https://community.traefik.io/t/possible-to-use-without-containers-simple-web-proxy/11918/4?u=wollomatic
Regards,
Wolfgang