The LTPP3 ships configured with a fixed IP address such as 192.168.75.208 by default, making it unlikely that it will automatically connect to your network. A reset of the Ethernet adapter's configuration (e.g., following a firmware reinstallation) could also leave the device disconnected from your network. In either case, you will need to either have the device automatically acquire an IP address from a DHCP server or manually assign it one.
To enable DHCP, you'll need to edit the /etc/systemd/network/wired.network file.
First, connect to the device via the serial console and log in.
To edit the file, start the Midnight Commander editing software with the command mc (you can also use any command-line interface editing utility that you might prefer, but it is unlikely that it will be installed at this point). Use your keyboard or mouse to navigate to /etc/systemd/network/ in the left-hand pane, select wired.network, and select Edit.
Delete the contents of the [Network] section and replace it with "DHCP = ipv4" (without the quotes) to enable DHCP for the Ethernet adapter. If DHCP on your network is handled by a system running Microsoft Windows Server and you need to explicitly require MAC-IP address assignment, add "ClientIdentifier = mac" on the next line.
Save your changes and then exit Midnight Commander.
Run systemctl restart systemd-networkd to apply the changes. Next, run ip address show and verify that the device has acquired an IP address from the DHCP server. Note that getting an IP address can sometimes take up to a minute, in which case you might need to wait before checking whether one has been assigned.
Manually assigning an IP address
The process of manually assigning your device an IP address is similar to enabling DHCP, in that you need to edit the /etc/systemd/network/wired.network file.
However, instead of clearing the [Network] section's default contents, replace the default IP address and gateway with your desired values.
After saving your changes and exiting Midnight Commander (or whichever editing utility you're using), apply the changes with the command systemctl restart systemd-networkd.
Via the Web Interface
You can also configure your device's networking settings via the Web Interface, but it's a relatively more complicated process than via the serial console if you're doing so for the first time.
For this procedure, you'll need a network patch cable to connect the board to your computer, whose IP address will need to be manually set to be on the same subnet as the device. Access the Web Interface by navigating to the device's default IP address, log in, and select Network in the left-hand menu. On the Network page, you'll be able to adjust the device's networking settings as usual.
Note that changing from DHCP to a fixed IP address or vice versa will disconnect you from the Web Interface. To resume using it, navigate to the device's new IP address.
Remember to reconnect the device and your computer to your network, and return your computer's networking settings to their state before this process.