256QAM was introduced in LTE Categories in Release 11, there wasn't many Devices released with CAT11, but quiet a few with CAT12 so any Device that is CAT12 or above should be capable of 256QAM support.
However, seeing that EE have there own Take on this model and Supporting Firmware I would prefer EE to confirm themselves on the 256QAM support for this EE Branded Router.
To save time for anyone trying to find public information on the hardware, the closest I can currently find is the B612. (At least a similar generation of product.)
My feeling is that to be a Cat 12, it would need to be based on the Balong 750.
