That seems odd. I saw for myself Friday that battery life is not as good when I'm using AT&T's network vs. WiFi but it didn't seem dramatic.
The difference is huge for me. On my Note 8 which is 1.5 years old now and still on the original battery, I have 2 days 0 hrs 8 minutes using my T-Mobile sim and the battery is at 55% with two hrs screen time. on AT&T I would barely get 1- 1.5 days out of one charge.
My iPhone XS Max is even more impressive on T-Moile. I last charged my XS Max Tuesday morning and it's just now dropped to 20% battery. ON AT&T Iget about 1.5 days with very little usage.
I have to use WiFi calling at my house on AT&T because I get no signal here in the middle of Denver and it's been that way since 2007 when I first tried them out.