Faraday cages are passive and not illegal. Jamming is.

>although if you use a busted microwave from the 80s it gives you good plausible deniability.

Not every radio runs off 2.4G, the frequency that microwaves would affect. Even for wifi there's 5ghz and 6ghz bands. For cellphones there are far more: https://en.wikipedia.org/wiki/5G_NR_frequency_bands

"just"