The computer that will act as a server needs also a static IP in the local network.
You can see your current (local) IP address by typing $ifconfig.
At L200 such addresses look like 192.168.178.XXX, and typically they are assigned automatically by the router when a device connects to the network through a protocol called DHCP.
So, for our server shouldn't receive a random local address but keep always the same. In our case we chose the address 192.168.178.200 (it is safer to choose as an address one that is not in the same range with the address assigned by DCHP).
To fix the local address of your server-computer for an ubuntu server you can follow these guidelines: https://www.ntweekly.com/2021/03/18/set-static-ip-address-on-ubuntu-server-20-04/