I would like to inform you that you are able to set WAN to work as LAN in both legacy and RutSO firmwaresfirmware.
Instruction on how to set WAN as LAN in RutOS could be found in a link bellowbelow:
https://wiki.teltonika-networks.com/view/Setting_up_WAN_as_LAN
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In order to avoid any issues, we recommend to use using RUT9XX_R_00.06.00.2 or a higher firmware version.