Dynamic DNS warn error

Labels WAN interfaces like add_local_fqdn instead of your ISP assigned default which may be obscure. WAN is inferred from config dhcp sections with option ignore 1 set, so they do not need to be named WAN
:!: add_wan_fqdn available since 18.06

From: https://openwrt.org/docs/guide-user/base-system/dhcp#all_options