How can I link a static page to a subdomain on the Apps Platform?

I want to prefix this by mentioning that I’m very new to development, and this is a “learning project” for me, so my question might be dumb.

I’m currently using the app platform to host a Flask website. I have a custom domain linked via a flattened CNAME record. I’ve added the subdomain for api.website.com via another CNAME record.

Usually, with Flask, I would do the following to load the root page for a subdomain:

@app.route('/', subdomain="api")
def api_load():
    return render_template("page.html")

When visiting the site using the subdomain, flask does not acknowledge it, instead just displaying the default root page.

I think that my issue is being caused by using a CNAME record, however, I can’t reliably see a way to use an A record because I can’t find a static IP for my app.

The only other solution I can see is to create another separate app, and just redirect the subdomain to that instead, but this would double my costs and I’m not eager to do that.

Am I missing something, or is this feature simply not available on the apps platform?

Thanks in advance for your time.