I'm a little late to this thread; I just found it after searching for the same problem. Unlocked Samsung Note 9, no WiFi calling on AT&T.
I still have an older AT&T Microcell that allows you to route mobile traffic over WiFi. I used it 10+ years ago. We are in the country and had a weak mobile signal at the time.
I just plugged it in and confirmed that it works! I went from 1-2 bars, to 4 bars, on my Mobile signal.
I paid $100 for it years ago. You should be able to pick one up on eBay or similar pretty cheaply. It looks like they sell for $20 to $30 used.
Once installed, If you look under Mobile in Connections on your phone's menu, it should say "AT&T Microcell" when you are connected to it.
Here is an AT&T link about the Microcell:
https://forums.androidcentral.com/e...w.att.com%2Fatt%2Fmicrocell%2F&token=Y8brwRuw
I am not paying any fee to use it. I think they used to charge me $5 or $10 per month 10+ years ago?
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I was actually using the Microcell until my son left for college a couple of years ago.
Even though it has theoretically been replaced by WiFi Calling, he would often use a lot of mobile data by forgetting to turn WiFi on while streaming video or gaming. The Microcell forced that traffic to WiFi even if he was "supposedly" using mobile data. Huge help with older 15 GB shared plan.
Also, FYI: I had a Verizon (unlocked) Samsung Note 8 before I bought the unlocked Note 9. When they pushed out the Android Oreo 8.0 Over Air Update, they installed the AT&T version of the release software That enabled WiFi Calling on my Note 8.
Good luck!
Michael