Cannot register a third-party cell phone signal booster with AT&T

We purchased a third-party cellular signal booster to support a medical alert device for my father.  After setting up the booster to confirm it will generate a boosted signal strong enough for the device, I went to register the device (per FCC regulations) with AT&T. The only instructions I could find required me to install AT&Ts booster app on my phone. After doing that, the app asked for the device's serial number. When I typed it in (several times) the app returned 'invalid serial number'. I then learned the app is to support AT&T's own cell signal booster. Online, I found a listing of devices that can be registered with AT&T, but this brand (PhoneTone or PhoneToneTech) and model (AN-L65AV) aren't options. How can a person register a device with the FCC through AT&T, as instructed and expected, if there are no means or tools available to do that?