How you set it can vary depending on what you’re running. Linux is all about choice, and choice means multiple ways to do a thing. Places to look:

• raspi-config - catchall CLI tool on Raspbian, not sure if it covers both of the following, but it’s a good option to start with
• /etc/network/interfaces - the Debian way to do things
• Network Manager - usually used for desktops, but can be used by servers; if the first two don’t work, try this

But you could also have installed something different. If you post your OS and version and what you’re running on it, I can give better advice.

That said, normal networking rules apply:

• don’t set a static lease in your DHCP range, or you could get conflicts, which can look like it’s not working
• consider using DHCP and setting the static lease through your router; use the MAC address and you’ll never need to mess with network settings on the Pi
• if you set on the pi, make sure everything is correct (netmask, gateway, interface)