best phone for ATT new generation 4g and 5g networks

You need to buy off the list.  Just correct bands is not enough. 

Non carrier branded phones that will work on AT&T

The short list is:

LTE on AT&T:

                        Band 2 (1900MHz)

                        Band 4 (AWS 1700MHz/2100MHz)

                        Band 5 (850MHz)

                        Band 12/17 (700MHz) - backbone

                        Band 14 (700MHz) - public safety channel

                        Band 29 (700MHz) - supplementary channel for downloading

                                                             only

                        Band 30 (WCS 2300MHz) - supplementary band

                        Band 66 (AWS-3 700MHz/2100MHz)

5G on AT&T:

                        Band n5 (850MHz) - Low Band

                        Band n260 (39GHz) - mmWave

As far as VoLTE (what AT&T calls HD Voice), the phone needs to be on AT&T's whitelist if you are considering an Android version. Any iPhone from the iPhone 7 upwards will work just fine.

Thank you for the detailed answer.

Just in case to make it clear. For example, I would like something like the last one, Xiaomi, Oppo, etc. They almost have a full support AT&T bands excluding band 14, will they work in AT&T network or they will be blocked, because not from whitelist ?

I just would like to understand how it works, all non-whitelisted phones will be blocked, no matter if they support all bands and HD voice technology or not.

@bl00dcat

I posted a link to the AT&T list of compatible phones.  I must emphasize several times because you are not getting it! 

Use the link.  Read the PDF. You have listed two Brands which are not on that list. Do not buy them they will not work. 

If the phone is not listed by model number on that PDF, it will not work on AT&T. It will not matter if it has all compatible bands.  

Stick to the list.

There are software requirements.   Non listed phones will be blocked from the network 

@bl00dcat 

Hello. I won't pretend to be an expert or anything here. 

My knowledge on this issue is from dealing with trying to get international devices purchased from third-party vendors compatible with the network.

I've dealt primarily with Samsung phones, and a few Motorola phones. 

The information @formerlyknownas  has provided you here is correct... according to At&t. 

They created two whitelists of phones that they claim are the only ones guaranteed to work on the network. 

For the easiest compatibility match, you should choose one from the list. 

The bands @OttoPylot listed are also correct, however there are workarounds for devices not on the whitelist... it's your choice to spend your money on phones knowing At&t will not allow them to work on the network.

But it's not because of any software or certification problem. 

Samsung Galaxy A70 not on either whitelist, cutoff just before Feb 22, and is currently fully functional with the network, even using the HD Voice feature for calls. 

However I purchased the phone in late 2019-early 2020, and I didn't know about the whitelist. I was aware of the pending 3G shutdown by the major carriers. 

@bl00dcat 

In addition to trying to match as many primary frequency bands as possible, you should also be 100% certain that the device is at least 4G/LTE/VoLTE capable... as well as HD Voice capable.

Those features are built in from the manufacturer.. and you can see that information online.

Having mentioned that I dealt primarily with Samsung phones, I'm not sure on the specs of those brands. 

@Nicolas81 thanks for sharing your experience! This is exactly what I was looking for.

@bl00dcat

3g is not off everywhere, yet.

  I am very skeptical an A70 is working with HD voice unless it's been flashed with att software.

Samsung is still adding phones.  But A70 may not make the list

@bl00dcat I appreciate your response and no problem. I am here for the same reason... to ask questions, get help/learn, and help others in any way I can.. if I can. 

If you decided that you want to try the workaround I'm using... just let me know. 

If not,  then cool, and good luck with your phone issues, and let me know if you find another solution. 

@formerlyknownas 

While I realize that your last comment on this thread was not meant for me... and I admit that most of the links sent to me here are over my head in terms of technology... I highly doubt that Samsung is doing anything to make this phone compatible with At&t, or any other carrier's network. 

I received another notification from At&t about upgrading my phone (Galaxy A70), and the service being cutoff roughly two weeks ago .

The workaround I'm using hasn't stopped working since February 22, works using the HD Voice feature for calls (when applicable), and isn't flashing.

However, should the service be suspended... I will post here to let others know.