Correct, you cannot lock band 38/40 using MR600. But I think it will prioritize whichever bands that provides LTE-A.
An interesting feature of the modem is that it connects to two bands at the same time, presumably for better reliability. In my case it's band 40,40.


(Please ignore the 0Mbps speeds, clearly a glitch happened there)
As far as speeds go this is what I got. Normally around 50-60Mbps, and can go to 80+Mbps off peak. Not too shabby for the price. I'm satisfied.

Huawei B618s-65d is still going be to much better than MR600, I've seen some people get over 100Mbps with that. But you'll have to cough up the money if you don't take the contract.