> Modern smartphones are generally both more locked down and also don't come with an external antenna option.
There are USB On-the-Go compatible SDRs [1] that you can hook up to an Android phone that cost like $50 (don't know if there are any that would work with iOS though).
[1] https://www.nooelec.com/store/sdr/sdr-receivers/nesdr-nano-t... (this one doesn't support wifi frequencies, but this was off the top of my head)