USSD for Mudi GL-E750

I tested your suggestion, without quotes:

AT+CSCS=GSM
Error

If I do it with quotes:

AT+CSCS="GSM"
OK

On the other hand, when entering the commands:

AT+CSCS?

+CSCS: "GSM"

OK

AT+CSCS=?

+CSCS: ("IRA","GSM","UCS2")

OK

Could it be that the right syntax for the USSD (AT+CUSD=1,"*444#",15) is different?
It’s worth to note that prior to the CUSD command, I enter AT+QCFG="nwscanmode",2 in order to bring the module to 3G (in many cases the USSD queries don’t work with LTE networks).

Thanks in advance for your help!