How to seperate wan from lan which they uses same port on openwrt x86?

I have pc installed with openwrt 19.07.1 that has only one ethernet port, my goal is to setup wan (pppoe) along with lan that forwarded to dsl router with disabled dhcp, is it possible to achieve that using vlan?, i tried to set wan and lan on same interface eth0 and disabled bridge on lan it works but i am getting packets loss from router ip from 40% to 60%

Thanks in advance