Hi, in stock firmware, following configuration make B1300 wan as vlan 88.
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 'auto'
config interface 'lan'
option ifname 'eth1'
option force_link '1'
option type 'bridge'
option proto 'static'
option ipaddr '192.168.1.1'
option netmask '255.255.255.0'
option ip6assign '60'
option ieee1905managed '1'
config interface 'wan'
option proto 'dhcp'
option ifname 'eth0'
config interface 'wan6'
option ifname 'eth0'
option proto 'dhcpv6'
config interface 'wan88'
option ifname 'eth0.88'
option proto 'dhcp'
config switch
option name 'switch0'
option reset '1'
option enable_vlan '1'
config switch_vlan
option device 'switch0'
option vlan '1'
option ports '0t 1 2 3 4'
config switch_vlan
option device 'switch0'
option vlan '88'
option ports '0t 5t'
config switch_ext
option device 'switch0'
option name 'QosPtMode'
option port_id '1'
option mode 'dscp'
option status 'enable'
config switch_ext
option device 'switch0'
option name 'QosPtMode'
option port_id '2'
option mode 'dscp'
option status 'enable'
config switch_ext
option device 'switch0'
option name 'QosPtMode'
option port_id '3'
option mode 'dscp'
option status 'enable'
config switch_ext
option device 'switch0'
option name 'QosPtMode'
option port_id '4'
option mode 'dscp'
option status 'enable'
config switch_ext
option device 'switch0'
option name 'QosPtMode'
option port_id '5'
option mode 'dscp'
option status 'enable'