I also have DIGI you can do via webgui using LUCI I followed this post: How to configure WAN VLAN on MV1000 Brume? - #3 by l3db3tt3r
You can use my config file to compare it with yours
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 ‘XXXXXXXX::/48’
config interface ‘lan’
option type ‘bridge’
option ifname ‘lan0 lan1 usb0’
option proto ‘static’
option netmask ‘255.255.255.0’
option ip6assign ‘60’
option hostname ‘GL-MV1000-d1d’
option ipaddr ‘192.168.8.1’
config interface ‘wan’
option hostname ‘GL-MV1000-d1d’
option metric ‘10’
option proto ‘pppoe’
option username ‘XXXXX’
option password ‘XXXXX’
option disabled ‘0’
option ipv6 ‘auto’
option ifname ‘wan.20’
config interface ‘wan6’
option ifname ‘wan’
option proto ‘dhcpv6’
option disabled ‘1’
config interface ‘lan0’
option ifname ‘lan0’
option macaddr ‘XXXXXXXX’
config interface ‘lan1’
option ifname ‘lan1’
option macaddr ‘XXXXXXX’