How do I point a subdomain to a different nameserver use Google Domains?

I discovered this question: Can I use different nameservers for different subdomains?

And I tried it out using Google domains.

I currently have a site hosted with AWS. However, I’d like to use Siteground to host a subdomain.

How can I do this through Google domains? According to the answer I posted, you just have to update an NS record on Google domains with the desired subdomain name and the nameserver. However, I tried this and it is not working.

For example, in Google domains I’m using 5 AWS nameservers: 

and in the custom resource records section, I have a NS record set up like this:

Name    Type    TTL     Data dev     NS      5m                

On siteground, I have a WordPress site installed on the domain

What else do I need to do to make it work?