You only need to modify, /etc/config/network files to configure LAN port as WAN port.
config interface 'loopback'
option ifname 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'fd74:c775:cee0::/48'
config interface 'lan'
option type 'bridge'
option proto 'static'
option force_link '1'
option netmask '255.255.255.0'
option ip6assign '60'
option hostname 'GL-AR750-1b4'
option ipaddr '192.168.8.1'
config interface 'wan'
option ifname 'eth0'
option proto 'dhcp'
option hostname 'GL-AR750-1b4'
config interface 'wan2'
option ifname 'eth1'
option proto 'dhcpv6'