Your router has a public IP address (that's how it can talk to Google and RaspberryPi.org). I'm going to assume your Raspberry Pi has a fixed IP address of 192.168.3.14 (to make my examples easier).
Your LAN uses a private network which is usually 192.168.0.0 to 192.168.0.255 or 192.168.1.0 to 192.168.1.255 with your router at
http://192.168.0.1 or
http://192.168.0.254 o
http://192.168.1.1 or
http://192.168.1.254.
To bridge the gap between your private network and the big wild and ugly public internets you need a process called port forwarding active on your router.
That says if a random punter from out in the internets connects to
http://198.51.100.1:80/ [dummy address] or connects to
http://www.example.co.uk:80/ your router will "port forward" that request to your Raspberry at
http://192.168.3.14:80/ and your web server will send back an index page.
The first step is to look at your router's configuration pages. You can find the address for that from your Raspberry with an
ip route | grep default command.
Next step is to search google for "<myrouterbrand> port forwarding" with the right brand and model in that search. For example:
https://www.google.com/search?q=BT+Hub6+port+forwarding
You also need to discover your public IP address. Which you can get from
http://ip4.me/ that will be needed for your purchased domain.
In some parts of the internets your ISP will have your public IP address behind a carrier-grade NAT. If that is the case then you can't run a public web server from your network. That is almost guaranteed if your internet connection is through mobile phone tethering or through an "internet on legs" dongle.
Before doing ANY of this stuff make sure you've done some of the security basics at:
https://www.raspberrypi.org/documentati ... ecurity.md
because if you don't do that it's about four minutes before your Raspberry Pi WILL BE compromised by a hacking attempt.
