I have had some luck with figuring this out. I've patched my CommCenterClassic using a patch from *****.
With CommCenter patched you can load unsigned IPCC's.
After some digging around I've found that two files in the IPCC deal with LTE:
overrides_N41_N42.plist & overrides_N41_N42.pri
I've added the files from the AT&T carrier bundle to my carrier bundle. This gives me the "Enable LTE" switch and my phone displays the LTE badge when in an area covered with LTE. Unfortunately when I try using data I get a pop up saying "can't activate cellular data."
I suspect there is just some configuration I am overlooking. If I make anymore progress I will post an update here.