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'