Can I configure RM1261 to operate in the 9xx MHz range via AT commands?

No, it is not possible to configure the RM1261 module to operate in the 902-928 MHz range via software or AT commands. The RM1261 and RM1262 are hardware-specific, and users must select the correct hardware (RM1261 or RM1262) based on the regional frequency band in which they need to operate. Switching between these frequencies via firmware or software is not supported.

You can find more information in: EZ-GUIDE-ATInterface-RM126x_v1_6.pdf where you can find: 

ATS 611 command that sets the region of operation. 

 Possible values are as follows: 
         0: Unconfigured 

RM1261 only: 

        1: EU_EU868 
        2: UK_EU863_870 
        8: TAIWAN_AS923_1 
        9: JAPAN_AS923_1 
        11: INDIA_IN865_867 

RM1262 only: 

        3: USA_CANADA_US902_928 
        4: AUSTRALIA_AU915_928  
        5: AUSTRALIA_AS923_1  
        6: NEW_ZEALAND_AS923_1 
         7: NEW_ZEALAND_AU915_928