Quite cheap the price, probably should've picked this one

Yes NATed. My Xbox is extremely slow.
But I heard there's a fix. You can Google.

Which cctv you're using?
The ones that connect to own server like xiaomi cameras - This one no problem
Standard cameras like hikvision etc with decoders that needs portforwarding - This one needs vpn or ipv6

if your CCTV need to do port forwarding in order to access from external, then you need public ip from digi, private ip won't work.
if your CCTV is p2p, you no need to assign any ip address, you just need to match the ID and password in order to access.

what my suggestion is if digi is not giving public ip to access from external, like my alarm system which is using android apps to control, i just install team viewer in a desktop which is connected to the wifi in my house and install remix os player to control it from outside.

test speed at http://www.speedtest.net, can see IP address

Same, I had to use some specific setup for the vpn portforwarding to access my ipcamera
Basically:

1) WS880 (Running xwrt) connect to VPN server
2) DNS - setup to redirect vpn ip address
3) Portfoward as usual to cameras/services
4) There's an also additional setup (script) for portforwarding with vpn.

I'm sticking to this setup until I can get ipv6 working

I'll pm you the instructions once I get back home, still at kampung now
Do you have asus router or any custom firmware like ddwrt/gargoyle running on your router?

Bought from taobao?
Seems like a nice modem

Nice, have you tried setting ipv6?
Maybe you can try to remotely access your ipcamera without portforwarding