Edit the file /etc/config/network, change wan section as below,
config interface 'wan'
option ifname 'eth0'
option proto 'static'
option ipaddr '192.168.1.38'
option netmask '255.255.255.0'
option gateway '192.168.1.1'
option dns '192.168.1.1'
Restart network by
/etc/init.d/network restart