I know I originally posted this in another thread as a response, but for the sake of making things easier to find for new folks coming here, I thought I'd consolidate your one-stop-shop for getting your unlocked, unbranded, Galaxy Note N7000 working properly on AT&T.
To get your Note fully functional, and running optimally on AT&T, it's not quite as simple as just popping your SIM card in and off you go...
The three basic things you need to do are as follows:
1. Make sure you have the correct account provisioning.
2. Configure and use the correct APN('s).
3. Disable Fast Dormancy.
I'll cover these in more detail below, but first of all I need to clarify something: the Note supports 21mbit HSPA+, which if you want to call that 4G, OK fine. Personally I try to avoid marketing catch-phrases like "3G" and "4G" altogether, so henceforth I shall refer to the Note's mobile connectivity by what it actually is: HSPA+
Now, before you're able to fully take advantage of HSPA+, you'll need to make sure that your account is properly provisioned for that feature, otherwise you'll run into a virtual speed limit which will keep your device from reaching its full speed potential. So, in order to check that you have the correct provisioning on your account, you should call in to customer service and make sure you're on a proper 4G (not LTE 4G either!) plan. Ask them if you have the correct SOC code. In my case (grandfathered unlimited data user), my SOC code should be "HSDPPB." If you're on a tiered plan, it'll be something else. Either way, you need to be on a 4G plan of some sort.
"Ok, how do I do that then?"
Easy. Go to you local store and get the IMEI from a similar Samsung device (from a connectivity standpoint anyway), like an Infuse 4G, or a non-LTE Galaxy S2. From the dialer, enter in *#06#, and the IMEI should show up. Make a note of that number (hehehe). You can probably find another IMEI on the internet, too, but either way you need that. When you call in to AT&T customer support, just tell the rep that you've upgraded your device to a *insert name of device from which you got the IMEI* that you bought on eBay or something, and you want to make sure that you're on the correct plan. Provide them that IMEI, and now your account should be provisioned for HSPA+ (YAY! WOO!).
Now that that's out of the way, you need to configure your APN or APN's if you're using more than one (this only usually applies if you're grandfathered into an older plan). Go to Settings => Wireless and network => Mobile networks => Access Point Names. From here, tap the menu button and select "New APN."
Populate the fields as follows (Case sensitive!):
Name: Doesn't matter, you can call this whatever you want.
APN: Phone
Proxy: <not set>
Port: <not set>
Username: <not set>
Password: <not set>
Server: <not set>
MMSC:
http://mmsc.mobile.att.net
MMS Proxy: proxy.mobile.att.net
MMS port: 80
MCC: 310
MNC: 410
Authentication type: <none set>
APN type: internet + mms (if you're on a tiered plan) or just internet (if you're on a legacy grandfathered plan).
This APN info above is for achieving the highest possible speeds over HSPA+
The next APN generally offers slower data speeds, but depending on your area may not be any different in terms of speed after all. This is the old legacy Cingular APN, and you need to input these settings if you're on a grandfathered legacy plan, otherwise you may not be able to send/receive MMS properly.
Name: Doesn't matter, you can call this whatever you want.
APN: wap.cingular
Proxy: <not set>
Port: <not set>
Username: <not set>
Password: <not set>
Server: <not set>
MMSC:
http://mmsc.cingular.com
MMS Proxy: wireless.cingular.com
MMS port: 80
MCC: 310
MNC: 410
Authentication type: <none set>
APN type: internet + mms
NOTE: For grandfathered users who wish to exclusively use the new "Phone" APN for their data instead of being able to switch back and forth between the two, you can set the wap.cingular APN type to just MMS only, and everything should still work fine.
Congratulations! You should have now successfully set up both your account and your APN's properly for making your Note fully functional on AT&T! You should now at the very least be able to get onto the internet and browse etc. Pat yourself on the back!
But wait! There's more!
You still have to disable a feature on your Note called Fast Dormancy. Fortunately, this is really quick and easy to do. But first, what the hell is that, and why do I have to disable it? Well without getting all technical, simply put, it's a feature which allows a device to very quickly switch from an idle data state to UMTS/HSPA/HSPA+ and back to idle again, and generally work more efficiently, thus improving performance, and saving battery life. Problem is, in order for this feature to work, BOTH the device and the network that device is on need to support it. Unfortunately, AT&T does not currently support Fast Dormancy. As a result, by having fast dormancy enabled on your device, it'll have the complete opposite effect than what it was intended to have. Basically, your battery life will suck more, data connections will be erratic, slow, and unreliable, and you're effectively wasting data overhead on a feature that, for AT&T, just doesn't exist. Disable it, and your Note will fly like it was intended.
To disable Fast Dormancy, from your dialer, enter in *#*#9900#*#*
This should pull up your SysDump menu. There, close to the bottom it should indicate your current Fast Dormancy state, enabled by default. Select that option, and it should indicate that Fast Dormancy is disabled. Exit and enjoy! Reboot the phone for good measure though. You generally don't have to, but sometimes it irons out any issues that may arise.
Hope this all helps! Enjoy your Note on AT&T!!