Dunno the details about Teltonikas stuff, but running "official" openwrt on a RUT955, it is not required to create a new interface for the wifi to the port, but simply to change the SSID/password on the existing interface (client). Implemented it myself in custom firmware. Although it is supported to run the same radio both as station and client, it first of all will reduce thruput. So, using a USB-dongle for the second WiFi is my standard practice.