The easiest way to switch SIM manually is through the router’s WebUI. You need to go Network > WAN. Select the Wan interface and click Edit this interface. In the General settings tab select Default SIM on or off.
More information on how to change default SIM through WebUI you can find here: https://wiki.teltonika.lt/view/RUTX09_WAN#General_Settings
The second way to change SIM manually by using CLI. You can edit the config file or use UCI commands. For example, you can try to change SIM by using these commands:
uci set simcard.@sim.primary=1
uci set simcard.@sim.primary=0
uci commit simcard
More information about UCI commands you can find here: https://wiki.teltonika.lt/view/UCI_command_usage