[AT&T Samsung Galaxy S8+ Guides, News, & Discussion] cadcamaro:
Before downloading and installing any drivers, I would recommend that you uninstall any other drivers that may conflict with the new Samsung drivers. Such as other phones like HTC, Motorola drivers etc. Anything, just to be sure. Once you remove them restart your system.

VZW Stock

http://updato.com/firmware-archive-select-model?record=F40B1761350E11E7963AFA163EE8F90B

Canadian Stock

http://updato.com/firmware-archive-select-model?record=23B1658C346A11E7963AFA163EE8F90B

Hacked Odin

http://www.mediafire.com/file/26modnk5c01n96h/Odin3_v3.12_PrinceComsy.zip

Samsung Drivers

http://developer.samsung.com/board/download.do?bdId=T000000117&attachId=0000000003

ADB Drivers from here

https://xdaforums.com/showthread.php?t=2588979

Just make sure to install it system wide.

This may also help, it might not. It's just an easy to find ADB folder that you can download and run from anywhere. Save it to your desktop, or wherever you like.

https://www.mediafire.com/?l3cvwuzik4v6455

If you had to install any drivers, go ahead and restart your computer now.

Those would be the exact setup I used when flashing my phone. If you run into any issues, as always try a different cable, OEM if you have it, and other USB ports on your PC. Hopefully this step by step will help get things working for you guys.

The first thing that you want to do is go ahead and factory reset your phone. Bring it to a complete stock phone, and make sure that you backup everything and anything that you need before wiping it. After you factory reset the phone, go through the initial setup and just skip everything. Let it activate, but don't bother with setting anything like a password, or extras like Verizon Cloud, etc.

Once you are at the home screen, go ahead into settings and enable developer mode on the phone and then go to the dev settings to enable USB debugging. With that done, switch back to your PC.

On your PC go ahead and open up the comsy version of Odin, and load all the files from the Canadian XAC folder. Just extract the main archive to get to the files within. Also make sure for this step you select the regular CSC file and not the Home_CSC file. The AP always takes awhile to fully load so give that a few minutes and find your ADB folder, or the folder that you downloaded from above.

Make sure that the ADB folder is extracted somewhere easy to find, mines just on my desktop. Then hold Shift and right click on the ADB folder. This will give you some extra options, and the one you want to select is "Open Command Window Here" which will open the contents of the folder in your CMD window. Once you have it open go ahead and type "adb devices" for it to scan and load your phone. You may notice on your S8 a popup asking if you want to allow your device to connect to your PC, go ahead and check the "always allow" and then select yes to allow your phone to be debugged from your computer.

After you run the "adb devices" command it should start up your ADB driver and find your phone. Once it does, type "adb reboot bootloader" to get into download mode. By now the AP file should be loaded and Odin should be ready to flash. I personally didn't have to change any settings within Odin to get it working. If your version doesn't start the flashing after you click start, you may need to try a different cable or USB port out.

If your phone does start the flashing process all you need to do is wait a few minutes for it to finish the process. Once your phone boots up, go ahead and do the same thing where you skip through all the setup EXCEPT the part where you enter a password. Select the PIN security option, and just do something easy like 1111 or 1234, and make sure that you select the option to enter your PIN code before starting your phone. I'm not sure if this is absolutely needed, but I did it anyways as that's what I was told to do and it worked.

Now that you're on the XAC firmware, you'll notice it looks completely unlocked but you won't get any signal or service. Go to your phone app, and in the dialer type in *#2263# to get to the settings menu. From there you will see an option for Band selection. Click that and then within select all bands, and apply settings. I did this like two times just to be sure that I did it right and had enabled all bands.

Another thing, I'm not sure if the popup came up before or after I enabled all bands but I did get a popup to switch the phone to global mode. I remember that it didn't come up right away, so it might help to wait a few minutes to see if that pops up for you. This was with my Verizon sim card installed in the phone by the way. Maybe it will force that dialogue to popup if you try going into APN or your cell network settings. Or try connecting to WiFi, or turning WiFi off, etc. Just try a few things and wait a few minutes to see if that popup comes up. If it doesn't then it might not be important and I wouldn't waste more than 5 minutes if it doesn't come up.

From here, go back into settings and enable developer mode again. Once it's enabled turn on USB debugging again and allow your computer to connect to your phone if it asks again.

On your PC you should still have your Odin and ADB command prompt open. First on Odin clear all the entries, and select all the stock VZW firmware that you downloaded from before into each box. This time for the CSC option you want to select Home_CSC though. Once you have them all loaded up, go back to your command prompt and type in "adb devices" again to make sure the driver is running, and the phone is detected. Once you see your device, type in "adb reboot bootloader" once more to get into download mode. From there, just start the flashing process and let it go until it's all finished.

This time when you boot up your phone you will notice that it asks you for the PIN you entered when you were setting up the XAC version. Enter the PIN to decrypt your storage, and give it a few minutes to boot up. Once you see your new phone activation screen, you can go ahead and get the phone setup how you like it. Once you're on the home screen you won't have signal right away, but after a few minutes it will start to work. At least it did for me.

After you see you're getting service, and the LTE symbol try going online to see if your data is working correctly. Then send a test text message through the stock messaging app. Make sure you can both send a receive them. It helps to have a friend with you so you can see that everything, including MMS is working as well as phone calls.

That would be everything that I did, step by step. I hope that this will help a few people out as I tried being as detailed as possible here. If you do have any issues, I may doing it again step by step, and maybe include some pictures or a video to help you guys out.