Problem getting subdomain to return ELB instance

I'm not sure what I'm doing wrong, but following instructions doesn't seem to produce the result I need.
I have a domain registered at Route53: bornstorytellers.net
This points to an EC2 wordpress instance which is linked to via an ELB
It has an SSL certificate attached via ACM

This appears to be working fine (i.e. it resolves to https://bornstorytellers.net)

I have a subdomain registered in the hosted zone: online.bornstorytellers.net
This points to an ELB for an EC2 moodle instance.

However, the subdomain does not resolve to the EC2 moodle instance, instead it returns to https://bornstorytellers.net

Clearly I'm doing something wrong, but can't figure out what it is.

Can someone point me the way?

Thanks in advance.

K