Configuring it to Public IP couldn't be done from your end.

You need to call them to get it done, provided that they even got such service

if you paid for it, it is most likely already up. from within the network, you can visit whatismyip.com (or google any similar service) to check what the website detects your actual IP to be.

if whatismyip.com detects the same IP as promised by astro fibre, then you simply lack some basic understanding of the concept of public (static) IP. i suggest doing some reading. some basics:

1. public IP is the IP your router uses to communicate with the outside world. normally it changes every time you reboot your router

2. public "static" IP is a paid service - means your public IP never changes (reserved by the ISP), which is good for many different purposes

3. all devices connected to your router are isolated from the outside world; we call that a "private network" - each device gets a "private" IP

4. if you want all incoming traffic (from the outside world) to go to a specific device in your private network, for example a server, or a CCTV, then you need to configure "port forwarding" - basically telling all traffic coming to a specific port on your router (public IP) should be redirected to a specific device in your network (by private IP - it helps to configure this private IP to be static for that specific device as well)

The default admin login for astro RG21 router is:

user: rgw21admin
pass: rgw21admin

It's my friend's RG21 (Astro fiber installed only two months ago) and he says there is no option to input the PPPOE credentials.
I think he said Quick Setup only sets up Wifi