Hey,
Looks like there is a LTE CAT 6 in the works -
https://fccid.io/TV7R11ELT6L
I guess its a balancing act keeping costs low, hardware compact and software full of features. I'm running a RBM11G (1 GB port) with a LTE CAT 4 R11e-LTE with a mANT LTE 5o. The max download speed I have got from my provider is 50mb~, nothing to do with the mikrotik hardware.
I also have an STX LTE (10/100) and the max download I have got was 50mb~ too.
Are you coming close to the 100mb download on your STX-LHG LTE?