Rather than activating the SIM card that came with my Mini 4 (and needing to cal AT&T to cancel the Air 2 card), I took the card out of the Air 2 and placed it in the Mini 4.
I normally keep cellular data turned off (except when I need it) due to increased battery drain with it on while in a location with a poor cellular signal. Whenever I turn it back on, I have to reboot the iPad to get it to register with the cellular network. My Air 2 didn't require this.
Do you think this could be a bug with iOS 9 or a problem with the SIM card? It takes so much time to get a new card activated, old one canceled, etc., that I didn't want to go that route unless there's a good chance it will make things work correctly.
FWIW, both SIM cards are Apple SIM cards and are being used on AT&T.
I normally keep cellular data turned off (except when I need it) due to increased battery drain with it on while in a location with a poor cellular signal. Whenever I turn it back on, I have to reboot the iPad to get it to register with the cellular network. My Air 2 didn't require this.
Do you think this could be a bug with iOS 9 or a problem with the SIM card? It takes so much time to get a new card activated, old one canceled, etc., that I didn't want to go that route unless there's a good chance it will make things work correctly.
FWIW, both SIM cards are Apple SIM cards and are being used on AT&T.