The first time login setting on the latest RUTOS (7.x and newer) firmware is in the "vuci" file at /etc/config/vuci. The following lines needs to be set to a value of 0:
option firstlogin '0'
After making the change it is necessary to reload the "vuci" service by issuing a command: /etc/init.d/vuci reload
If your script executes specific commands via CLI (SSH), you can include the following commands in your script to make sure the first time login procedure never happens. I've also included the option for advanced mode to be enabled by default but if you do not need it you can simply remove the line for the advanced mode:
uci set vuci.main.firstlogin='0'
uci set vuci.main.advanced='1'
Alternatively, all of these commands can be executed in a single line:
uci set vuci.main.firstlogin='0' && uci set vuci.main.advanced='1' && uci commit && /etc/init.d/vuci reload
Let me know if this works for you. Also, please note that the path to the file with first_login setting is different on legacy firmware - it's at /etc/config/teltonika.