Not to mention ATT likes to lie to you which network tech you're using. There was quite a bit of a dust up when they were just putting the 5G icon on phones when they were connected to 3/4G signals.
The confusing terminology explained
www.androidpolice.com
Since your phone is only showing 5G and not 5G+ I suspect you're actually on the legacy LTE when it comes to speed..
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Some of these speeds listed are just not the best but, they do show the differences between the 3 networks to an extent. One thing that does help if you're getting marginal results if to put the phone into airplane mode for ~30 seconds and then back to normal an it will force the phone to reregister with the cell site and possibly pick different channels if they're available. Most cell sites have multiple channels setup depending on the direction of the antennas.
We produce independent coverage maps based on data crowdsourced from our global community of app users.These detailed coverage maps are available for free in the Opensignal app — download it now.
www.opensignal.com
If you want to test TMO in comparison a cheap option is a SIM from USMobile
https://www.usmobile.com/plans
Just make sure you get the right sim as one is TMO and the other is VZW. Tello is another option but, more of a long term setup with their plans / pricing. They both have a 5GB for $15 option though and that's sufficient for running a few tests. When you're dealing with 5G a speeditst will consumer ~500MB per test easily due to the higher bandwidth and no throttling on the apps.
Netflix / fast.com is alright but, ookla is the gold standard though some of the partner servers lack capacity and provide slow results sometimes. If you find one that's quick on your laptop / PC try to pick the same one on the phone.