On low-end FortiGate units, a DHCP server is configured, by default on the Internal interface:

These settings are appropriate for the default Internal interface IP address of 192.168.1.99. If you change this address to a different network, you need to change the DHCP server settings to match.

Alternatively, after the FortiGate unit assigns an address, you can go to System > Monitor > DHCP Monitor, locate the particular user. Select the check box for the user and select Add to Reserved.