you might want to take a look at softether’s site itself. it can act as a client to do exactly what you want, you just need to configure it to do so. the turris documentation will help you get it installed since it isn’t a native openwrt package.
https://www.softether.org/4-docs/1-manual/A.Examples_of_Building_VPN_Networks/10.5_Build_a_LAN-to-LAN_VPN(Using_L2_Bridge)