Hi Melissa,
We do have plenty of places in Montana without good cell phone/data coverage as well, especially on our reservations. We have run into this before. One of the ways we try to help with notifications is encouraging folks to sign up to our mass notifications through our alerting vendor. That way even if the WEA doesn’t hit their phone, our emails, phone calls, texts, and vendor app all pass along the same information (only in different ways.) The nice part is that hopefully their TV’s and radio’s were also interrupted with an EAS warning for your AMBER Alert.
I’m afraid I’m not familiar with the Wi-Fi splash screen.