The command you’re looking for is AT+URAT=x. This allows you to select a specific radio access technology.
Indicates the radio access technology and may be:
• 0: GSM / GPRS / eGPRS (single mode)
• 1: GSM / UMTS (dual mode)
• 2: UMTS (single mode)
• 3: LTE (single mode)
• 4: GSM / UMTS / LTE (tri mode)
• 5: GSM / LTE (dual mode)
• 6: UMTS / LTE (dual mode)
• 7: LTE Cat.M1
• 8: LTE Cat.NB1
• 9: GPRS / eGPRS