Streaming in vMix using a smartphone hotspot is not a good idea. It is simply too vulnerable. You might be able to get something going with several 4G networks combined using Speedify on the vMix laptop, but what you really want to do whenever possible is to hook up to the local network of the premise, using a (potentially long) network cable.

If you need wireless camera sources have a look at vMix call. Doing interviews this way is great, however, if you want to have people running a gunning with iPhones and have these sources directly in vMix, this currently is not possible. Take a look at Livestream Studio which works rather well with iPhone sources as long as you have the iPhones connected to a strong wifi (probably set up by your team before live).

If you need to be completely portable using 4G, take a look at AVIWest, LiveU and Teradek bonding. You will need to broadcast to a debonding server in the sky and then take the stream into vMix which will always introduce some latency.

Unless you absolutely need to be wireless go wired. NDI is now becoming relevant, but for most of us HD-SDI is still the way to go.