Hello,
Due to RUT950 hardware, it has two physical devices in terms of switch configuration, due to which the first three LAN ports share a single physical device, which is then split into three virtual devices. WAN port is separated and it cannot be configured in port based section.
In order to use WAN port as a VLAN port, it would have to be assigned a tag. For that, WAN port has to be configured as a LAN port first, which you have already done. Next, add an instance in Network -> VLAN -> Port Based of type 802.1Q for eth1 interface.
You would then need to create an interface in Network -> Interfaces section and assign a physical interface of a previously created VLAN instance.
Best regards,