App Engine phpmyadmin cannot connect to my Cloud SQL instance

[K S]

Here's the tutorial I've followed:
https://cloud.google.com/sql/docs/mysql/phpmyadmin-on-app-engine

Once I deploy, I see the phpmyadmin index page, asking for me to log in. When I try, I get following messages:

Cannot log in to the MySQL server

mysqli_real_connect(): MySQL server has gone away
mysqli_real_connect(): Error while reading greeting packet. PID=-1
mysqli_real_connect(): (HY000/2006): MySQL server has gone away

this is what I used for my instance name
myprojectID:us-west1:mycloudsqlname

I can't figure out what I'm doing wrong. I tried the both version on the doc and the latest doc.
I tried to allow unsecure connections for Cloud SQL, didn't work.
App Engine authorization under "Connections" tab is checked greed - Apps in this project: All authorized.
I'm only using Private IP address, associated with "default" network setting.
Nothing on logs.

Can someone confirm that the tutorial above is working?
Any help would be appreciated.

[K S]

I've found the solution: I had to enable the Public IP Address from Connection tab.
I still don't understand why I have to enable public IP address, when I want to connect between Cloud SQL and App Engine; they're all GCP services. I didn't have to enable Public IP address to connect Cloud SQL from my Compute Engine.
My Cloud SQL service is now ping-able, which makes me nervous too.

[Tiago (Google Cloud Platform Support)]

Hello, as relayed in the Private IP - Application environment requirements documentation, it's not possible to connect through private IP from standard environment. In order to only connect through private IP, you must use flexible environment instead.

[Kurtis Van Gent]

Hey KS,

Just as an FYI, you can grant your Serverless Environments access to a VPC with the Serverless VPC Connector (in beta).

Thanks,
Kurtis