After much research, I found that this nice (non-5G) phone (I bought in India but used in USA for the last 3 yrs), does not work with PureTalk USA, an ATT reseller, although it allows for VoLTE (calls it Enhanced LTE for both SIM slots).
I am unhappy that ATT / Puretalk is preventing this nice phone from being used on the network as part of their "phase-out of 3G support".
My Iphone X which also is not 5G, however, is supported.
Before I start taking other actions, I want to know from the ASUS people whether there is some setting on this phone (or an update) that I need to make/install to make it work.
The problem: Whether in Sim1 or Sim2, the phone goes into "Emergency Call only" mode and does not lock on to the network (in other words, No Voice network, No SMS). It worked great until May, when I had to leave the country for 3 weeks (So the sim went off the network). Upon returning, this problem arose.