AFAIK stingrays typically use 2G, since 3G and upwards have strong mutual authentication (i.e. the network authenticates the phone but the phone also authenticates the network). They just jam the 3/4G signal to make the target phone fall back to 2G.

Agreed.

I've never done it, but I suppose you could use 2 SDRs. One for setting up OpenBTS, one for jamming 4G/LTE.

There's NextEPC: https://github.com/nextepc/nextepc

Fake base station attacks do not work with 4G/5G, but a downgrade attack to 2G might still be possible.