Norway has a nationwide network of warning sirens that was built out during the cold war and that is tested regularly that could be used to convey a wider range of critical messages without the need for a radio receiver (during my childhood we heard these very regularly, but with the collapse of the Soviet Union, the test frequency was reduced significantly and now they're only tested twice a year).

However with the rise of the internet and decline of the printed phone book, which used to be distributed to absolutely every household, and that listed the (small) set of warning patterns the sirens would use (that included e.g. warning about air raids), they've now reduced it to only one signal to avoid depending on people to remember them:

"Important message - listen to radio"

...

I wonder how exactly they'll resolve that with a diminishing proportion of the population having "radio" that's independent of internet access being up.