Every ISPs process for installing a public server might differ but the main principles remain. Here is the guidelines for the case of Init7 / FRITZBOX: https://en.avm.de/service/fritzbox/fritzbox-5490/knowledge-base/publication/show/1638_Setting-up-a-public-IPv4-subnet-in-the-FRITZ-Box/
The most important step to install your own server is a permanent address where it can be contacted. That is, a fixed public IP address. In the case L200's fixed IPv4 address is 212.51.155.200 and subnet mask: 255.255.255.248 (this information will be provided by your ISP)
After your server is connected to the network with its permanent local address you will need to add it to the hosts that are exposed directly to the public Internet in your router's admin panel. In the case of L200 the admin panel is accessible at the address 192.168.178.1
In the case of L200 the corresponding action is available at the menu Internet->Permit Access->Add Device for Sharing__
There you can find your server through its name or add manually its static IP address (in our case 192.168.176.200)
Check that this address does not fall in the DCHP range: Home Network -> Network -> Network Settings__
After the server is added to the devices accessible from the public Internet one should determine the exact ports that will be allowed.
In the case of L200 the corresponding action is available at the menu Internet->Permit Access->Add Device for Sharing->New Sharing__
Some basic applications are: SSH at port 22 / HTTP at port 80 / HTTPS at port 443