AT&T might be fighting a losing battle against T-Mobile and Verizon in the US 5G arena

Luckily, the FCC gave Verizon and AT&T the perfect opportunity to catch up to Magenta in that particularly crucial field by holding its biggest auction ever, but although both carriers committed a boatload of cash to the mid-band 5G expansion cause, said financial efforts might ultimately prove largely ineffective.
Even AT&T knows it's game over already

The 5G for all tactic may work... temporarily
In the short term, that could prove an effective customer retention strategy, but when it comes to driving actual subscriber growth, Ma Bell's plan remains unclear. That's mainly because the third-largest wireless service provider stateside has such "scarce resources" that its top executives aren't even bothering to "make the case that theirs would be the best network in the era of 5G", as noticed by multiple financial analysts quoted by LightReading.
Basically, AT&T's limited mid-band 5G spectrum stockpile, even after spending more than $23 billion in that aforementioned recent auction, leaves the carrier with only one undesirable marketing option going forward. Instead of trying to paint its 5G network as the "best" choice for speed junkies out there, AT&T may ask you to settle for a "good enough" service in the near future.
Good deals are not enough to win this war
Of course, there's more to 5G than mid-band spectrum (namely, low-band and mmWave connectivity), but as already highlighted by T-Mobile's "Ultra Capacity" signal, this is truly the wireless industry's holy grail, delivering by far the greatest balance between speeds and availability.
Despite planning to splash up to an additional $8 billion on top of the aforementioned $23B to deploy the newly acquired spectrum and thus substantially improve its lagging 5G network in many places, analysts (and T-Mobile) simply don't believe AT&T's assets are rich and vast enough for the speed and coverage gap to ever be closed on a large scale.

The only other card Ma Bell can play is the value of its 5G services (hence the recent access expansion to a bunch of legacy plans), but in the long run, experts expect the appeal of such promotions (and device deals like free iPhones for new and existing customers) to "wane." Either way, that's a dangerous road to go down to when competing against T-Mobile, which often likes to throw free lines at everyone just for the fun of it.
All in all, AT&T may have painted itself into a corner with a mix of bad choices and objectively poor resources, and the only way out of this pickle is to hope customers will be fine choosing "good enough" 5G speeds and aggressive promos.