There are many factors that go into ul/dl speeds over wireless; you need a handful of tests for data to be useful. Are you both on the same carrier? Do you both have iPhone 7? Which models?
My recommendation is to call Apple Support and talk to engineering first. They will by now have some idea of the problems on AT&T and best solutions. Some people are being asked to send Apple some diagnostic data, while many others have had success with iOS 10.0.3 and 10.1. Apple Store is fine, but you're not likely to speak with a real tech expert there. They can swap your phone but many people report that didn't fix.
No one seems to know the cause of the ongoing problems:
- Some problematic hardware component that is bad in specific iPhone 7 handsets?
- Some iOS or carrier software problem interacting with some iPhone 7 hardware?
- Something wrong with backups restored to new iPhone 7 hardware?
- The vast majority of people never had a problem, yet some problems remain after many were resolved (without hardware exchange)
- Problems in US/Verizon seem to be resolved; remaining issues could be specific to the GSM-LTE iPhone 7