I have resolved my issue. The problem was that after installing and removing SixFab Core, the modem switched from QMI mode, which it was originally in, to PPP mode. I discovered this after connecting via minicom on my Raspberry Pi and entering the command AT+QCFG="usbnet". In response, I received +QCFG: "usbnet",1, indicating that the modem was operating in PPP mode. I switched it back to QMI mode by entering the command AT+QCFG="usbnet",0 and rebooted the modem with the command AT+CFUN=1,1. After rebooting my Raspberry Pi, the modem was once again recognized as /org/freedesktop/ModemManager1/Modem/0 [QUALCOMM INCORPORATED] QUECTEL Mobile Broadband Module, and I can now manage the mobile connection through Network Manager.