It is unclear how terminal v1.9b works, we at Teltonika do not use it but from what I understand it works like any other terminal and supports only serial connection.
USSD commands can be sent with AT+CUSD command. Please refer Quectel modem documentation for At commands usage: https://wiki.teltonika-networks.com/wikibase/images/4/48/Quectel_BG96_AT_Commands_Manual_V2.3.pdf#page=13
In your case it should look like this;
Also, you may find this guide useful: https://m2msupport.net/m2msupport/ussd-how-to-send-ussd-short-codes-with-at-command/