Galaxy s8 plus still dident recieve software update !!

It should but also the SIM card you're using needs to have the IMEI update on the network side so the device management can send the update.

@Arabjabba Doubtful, plus your device is going to get queued, in most cases, and won't receive the update while you're there, with that SIM in it.

It's "non ideal" to purchase an AT&T branded device for use on a non-AT&T network, for both updates and tethering, you're much better off waiting for non-branded devices (for the future).

Like @GLIMMERMAN76 mentioned about the BestBuy deal, you're probably better off to wait out the couple of weeks and have them do the update...

After that, well, you have to keep an eye out, for when the updates are released to AT&T firmware devices, and probably do the BestBuy thing again, and again...

Store updates (things like "instant tethering" and some others) will come over fine, because they're carrier-agnostic, but any features/fixes will have to come from AT&T or BB.  I'm not sure about (typically monthly) security patches, @GLIMMERMAN76 probably knows more about that model.

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@Arabjabba wrote:
So you are saying if i put att sim card i will get update immidiatly or got still wait ?

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@ArabjabbaEither is possible, but the wait scenario is most likely, part of it depends on where you are in the queue, and how many others are waiting at any given point...