says re: 4G
"Will the new TCU hardware work with the 4G network?
No. The new TCU hardware will be designed to connect to the current AT&T 3G cellular network." and
"Why is Nissan using 3G technology rather than 4G?
Nissan has studied how to integrate newer technology into your vehicle and developed solutions aimed at delivering a hardware update option with the least inconvenience to customers. The new TCU hardware will be designed to connect to AT&T's 3G cellular network. Nissan does not control availability of any cellular network and cellular network may not be available in all areas and/or available at all times."
Who knows if their chipset actually doesn't support it 4G or LTE or it's buggy or if they just aren't using the capability right now?
says "One change we’re making is phasing out our 3G network by February 2022. Once we do, 3G devices and 4G wireless devices that don’t support HD Voice will no longer work on our network."