Far as I know Foris does not (yet) provide VLAN settings for the WAN interface. If you are comfortable with ssh access try this in /etc/config/network (pertinent to TOS4.x and later)
config interface 'wan'
option proto 'pppoe'
option layer '1'
option username ''
option password ''
option peerdns '0'
option keepalive '6 10'
option ifname 'eth2.7'
option ipv6 'auto'
Mind enter the username and password. After saving the changes execute from ssh cli ubus -v call network reload or ifdown wan ; ifup wan.
If you are not comfortable with ssh use the LuCI interface instead. Check the logs about potential errors printed, either via LuCI or else from cli logread -f