never owned managed switch (yet). But under OpenWrt interfaces must be bridged like in my setup. Then on router side wan.9 is pppoe device and wan is bridged with other ports into br-lan
below my vdsl2 modem bridge config:
config device
option name 'dsl0.35'
option type '8021q'
option ifname 'dsl0'
option vid '35'
config device
option type '8021q'
option ifname 'lan1'
option vid '9'
option name 'lan1.9'
config device
option name 'br-wan'
option type 'bridge'
option mtu '1508'
option ipv6 '0'
list ports 'dsl0.35'
list ports 'lan1.9'
option multicast '0'