I have a developers unit and I am having the same issue as you. Please let me know if you find anything out as I will do the same. The other issue is that I cannot tether the phone either but I will deal with that one separately. I travel overseas a lot and I do not want a phone that is purchased from AT&T that is missing a lot of the international frequencies hence the reason for the developers unit (which has the AT&T LTE frequency?s).
I have called a few times to AT&T and have been given the run around. On my phone there are two APN settings (the first 2 are from AT&T) - AT&T LTE1-Lumia and AT&T LTE2-Lumia. LTE2 and the custom HSPA+ works and allows me to use data under H which I guess is HSPA? (sorry new to AT&T).
I have ran speed tests under AT&T LTE2 ? Lumia and the custom APN that I found here: (
http://forums.windowscentral.com/lumia-900/189221.htm) Although it is only HSPA+ settings, it will have to work for now as I am getting better speeds than what AT&T put as a apn on my phone (AT&T - LTE2 - Lumia). The search continues for a custom LTE apn setting to initiate LTE
AT&T LTE2 = (average) 3.3 Mbit/sec download 750 Kb/s upload
AT&T HSPA+ (custom apn)= (average) 6 Mbit/sec download 3Mbits/sec upload
I used an app in the market called Speed Test (my Speed Test) ? in case you want to run the same tests and let me know what results you are getting. Somehow we will find an apn (and fix the tether) that works.