FWIW, I just purchased an Orbi RBK-40 3-pack from Best Buy 2 nights ago and set it up in place of several Apple Airport Extremes I used to have around the house. (One unit is out in a detached garage so I can get Internet access out there.)
I'm currently running the same v.2.0.0.56 firmware you're using and we have two iPhones connected to the network right now. Both of them show up as using 5Ghz here. So definitely not seeing the issue you're having with them selecting the 2.4Ghz band instead.
Right now, the only devices I see on the network using 2.4Ghz are a couple of Ecobee 3 thermostats (which is to be expected), and an Android cellphone. (I'm pretty sure that's correct too, as that phone is a $20 ZTE smartphone purchased for one of our kids with a pay-as-you-go plan. It very well may not support 5Ghz wi-fi at all. She already complained to me once that it "refuses to connect to some wireless routers" like the one at my mom's house.)
I just ran the Ookla speedstest app on my iPhone 7 Plus to see what it said, and it came back reporting 203Mbps download speeds with our Comcast cable broadband.
