On the R201 only the WAN port can send and receive tagged packets.
For your scenario, you'll need to create two WAN connections - this KB article has some screenshots. There will be an existing WAN connection set up with VLAN disabled (i.e. use untagged packets), to which all ports and SSIDs are bound by default. You'll need to create a new WAN connection of type 'Other' and configure it with the VLAN you want to use for the guest network. Then at the bottom of the WAN configuration page bind it to the guest SSID