Indeed band 66 is not part of the physical hardware you have installed in your LHG LTE6 and cannot be ‘software fixed’
Your best bet if you want to use that band is to remove the LTE card inside your LHG and replace it out with a suitable card that supports band 66.
Something like a Quectel EP06-A will do band 66 and a bunch more
That said, replacing the card is not always straight forward as there may need to be some settings applied to the new card( as well the the routerboard / Router OS settings ) for the new card to work.
There are a few forum links on doing this, and you may need to get some additional items ( usb to mini-pcie ) board from ebay etc and depending on your skill level and patients you can usually get it working. Usually the main issues I see with putting in cards is newer minipcie card use usb-3 protocols which can sometimes be set to usb 2 mode, by way of software(on the card itself(e.g the use of a minipcie to usb board and fix it on a PC ) or via taping off specific pins on the usb3 minipcie-lte card to force it to run at usb 2.0 mode, and the routerboard itself which may only have a usb2 chipset, but some of the newer router boards have chipsets that are usb3 compatible, and makes like easier when using alternate 4g/lte & upcoming 5g cards.
There are other cards too like the Telit LM960 with I think is has very basic support in very recent updates.
Have a look at these posts for some ideas on what you need to do.
http://forum.mikrotik.com/t/quectel-ep06-e-and-wap-r-ac-rbwapgr-5hacd2hnd/131724/1
http://forum.mikrotik.com/t/quick-ep06-a-setup/147879/1