AT&T will shut down 3g in February 2022. To use their 4g and 5g, the phones need to support VoLte. Unfortunately, the new Zenfone8 does not seem to support VoLte over the AT&T network. (If it does, please provide setting guideline)
VoLTE should be enable for ASUS Zenfone 8, another user confirm this in another ZenTalk thread.
Please post a screenshot of your network settings and we can try to help you from there.
Please see the above screenshots.
ASUS Zenfone 8 is approved on the AT&T post 3G network (so the VoLTE I guess), but it is not approved for US AT&T's 5G. Keep that in mind.
I can not even use 4G. It keep saying I need to register VoLte.