Reddit reviews Midland WR-120 Weather & Alert Radio - with Biological Hazard Civil Emergency Message Fire Amber Alert Landslide Nuclear Power Plant - FM AM - 7 Weather

Look for a combo of devices / solutions.

Something like this
https://www.amazon.com/dp/B00CVT6ELW/ref=dp_cerb_1
for alerts.

Plus different plans; as u/petrolly called out, devices like the Moto G series do work on all carriers, but only single sim at a time.

Or the iPhone X onwards with eSIM + physical sim; you can enable two different plans from 2 different providers.

Or if you have multiple phones in your household - get plans from different providers, so that you can use whichever still works.

Couple it with a hotspot if you want, with a spare sim that can be activated as required (on a different provider, if you don't have all in the same family).

Add a simple AM/FM radio that's battery operated.

Keep in mind that cellular networks are probably the least effective way of tracking alerts; short range cells mean that if your nearest cell goes down (or there's a break in the network linkages) - you lose connectivity. You need to have alternate sources like the GPS network or public radio in addition.