Hi
The Billion 7800N is a bit funny to set up for remote access and I've never fully understood it so always been trial and error, but do have it working. There are two areas that seem relevant on first trying to set it up, these are in Configuration - Advanced, one is Web Access Control and one is Remote Access.
I think Web Access control is related to the internal network only, on mine it is disabled but I can still access the Billion from outside, be careful with this setting, I was messing with it trying to get remote access working and was able to lock myself out of the Admin interface completely! The terminology used for remote access on the Billion is confusing. Billion don't do UI interfaces very well (that's in addition to looking like a relic from the web circa 1997

)
The setting I found that is key is Remote Access, with Web Access Control best left alone and disabled.
If you enable Remote Access then you should be able to access the Box from elsewhere. You can also specify rules here so that only a range of IP addresses are accepted, for example you might put your work IP address here to access from work, however you will not know your IP address in advance from abroad so leaving no entries in here leaves it open to the world. Make sure you have a strong password set.
Note that if you have port-forwarded port 80, then to access the Billion remotely it moves to 8080 so you need to append port number 8080, so
http://mystaticip:8080
I quick way of checking if the port is now open is to use
http://www.grc.com and go to Shields Up, and test that port 80 (or 8080) is now open, then of course check again when you can from somewhere else to give it a try.
Do not forget to "Save changes" using the option at the bottom of the page, as otherwise on a reboot it might undo the settings.
Another more secure option is to set up a VPN, only useful if you are accessing from your own equipment to set up a VPN connection (i.e. I doubt in an Internet cafe you would have access to create a VPN connection). With the VPN you join your local network as though you are connected directly and then do not need any Remote Access settings enabled as you are local via the VPN.
The VPN works from Android devices and PCs.
To set up a VPN on the Billion, Configuration - VPN - PPTP, Set to Enable, WAN port default, Auth Type MS-CHAPv2, Encryption Length Auto, Peer Encryption Mode only stateless IP address start use the default, time out 0 mins. Then create an account, enter a name of your choice, Tunnel is Enable, Username and password of your choice, Connection Type Remote Access, Peer Network IP and Netmask leave blank. Incidentally the VPN is a good to use if on a public Wi-Fi connection as all traffic is encrypted over the VPN and sent to your home router and joins the Internet from there, avoids anyone snooping on your traffic when on unsecured Wi-Fi connections. Also some content such as BBC online content is often blocked if you are not in the UK under the assumption you don't have a UK TV licence, but the VPN will present you to the BBC just like you are at home in the UK.
With an Android device, go to Settings and More, click VPN and add, Enter username and password as set above, type is PPTP, server address is your static IP address, click Advanced, and for DNS enter your routers IP address. You should be able to test this even if connected already to your Network via Wi-Fi. With a Windows PC, just add a new VPN connection and follow the wizard.
Hope that helps.
Regards
Phil