DNS config to point subdomain to another vhost

I have a digital ocean droplet where I have one server block serving a static homepage and another server block serving a meteor.js application. I'd like to point a subdomain at the meteor application, but at present the subdomain is loading the main static page.

my current DNS configuration is:

A        @    1.2.3.4
CNAME    www  mydomain.com.
CNAME    *    mydomain.com.
NS       ns1.digitalocean.com.
...

I thought by adding another CNAME it would point to my meteor application...

CNAME    foo   mydomain.com.

but when I visit foo.mydomain.com I get the main root domain page.

Any pointers appreciated.

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