They are using the same Unifi modem right?
Not at all. They are using "Skyworth RG21", an Astro Fibre branded WiFi 6 router. If you sign up Astro Fibre today, this is the only router model that you'll be getting.
Contact Astro customer service, tell them to provide you the PPPoE username and password.

If they've provided you the credentials, make sure you manually enter the username ends with "@astrobb.com.my".
Example: 123456789@astrobb.com.my

One less or wrong character will result in authentication failure (Internet won't up), so make sure you double check and entered it correctly.
Same goes for password, as it is in random alphabets and is case-sensitive.

Note: If you've manually requested/opt for Public IP, your username may have the "public" word in it.

Did you set VLAN ID properly?