Radio Broadcasting from Online Stream

FRS.

Bring your own WiFi in your car. 

The typical solution is multiple WiFi APs, like this 126 APs

I would research what do drive-in movie theaters do for licencing/compliance for RF, hopefully you could do the same.

I read on the internet that "The movie audio is provided with a local FM radio broadcast"

They must have a local radio FM station license?

or used a license-free https://wholehousefmtransmitter.com/who-else-needs-the-best-drive-in-theater-fm-transmitter-for-cinematic-sound-quality/

lol I don't know, but Id try to do the same as them. Other examples are parks having their own advisory FM station or traffic stations

50 W on FM, woo hoo!

Purpose

The purpose of these radio programming undertakings is to inform tourists in national or provincial parks and on historic trails about the features of the parks and trails, and to provide traffic control information.

Description

1. The undertaking meets all technical requirements of the Department of Industry (the Department) and has acquired all authorizations or certificates prescribed by that Department.
2. The undertaking operates between 525 and 1705 kHz in the AM frequency band or between 88 and 107.5 MHz in the FM frequency band.
3. The undertaking broadcasts with a transmitter power of less than 100 watts in the AM band, and with a maximum effective radiated power of 50 watts with a maximum transmitting antenna height of 60 metres in the FM band.

random quote: https://crtc.gc.ca/eng/archive/2003/pb2003-35.htm 

Of course this will vary region by region...

Some new cars, including Tesla, Ford, and BMW don’t have FM or AM radios. I was surprised to see this in my new car, especially as public warning messages are transmitted on radio. 

musictech.com/.../

Dumb idea.

I think those devices are banking on the exception for a personal area FM transmitter. They were popular 10-20 years ago as CD and MP3 player adapters for cars. They'd connect to the headphone jack and then transmit to the car's radio.

At least in the US, the FCC only allows those to have a very short range. So anything beyond 100 ft / 30 meters is highly questionable--regardless of what the manufacturer claims for the range.

I'd imagine in the original poster's situation, if they could get an internet connection that close, they wouldn't need to re-broadcast the stream.

Depending on your budget and whether or not this is an outdoor event, then you could perhaps take a look at some of the satellite phone data services such as Iridium GO. Looks like they can achieve 88kbps download, which is enough for an audio commentary, especially if you transrate the audio stream first before accessing it via satellite link. Voice data can be heavily compressed and thus  reduce the data rates required over expensive comms links.

If you are doing a group booking and can get enough space for the group at the event then there is the likes of Starlink Roam for satellite broadband. May also be an option if you have a campervan parked nearby.

You could perhaps use a PMSE license to do short audio links between two points, especially using directional antennas with line of site. This might be tricky however as different countries use different frequency allocations and licensing.

On-site this might be a use case for Bluetooth Auracast for public locations allowing the receiving device to broadcast to a number of local listening devices.

Other approaches may be to try improving your WiFi/mobile set-up for better reach. Directional external antennas and/or cellular multiplexing can sometimes help you reach beyond the main crowd.

About 10 years ago, I used to have a rig similar to this one: 

which bonded up to 6 external 3G cellular modems together using external mobile providers to be able to live stream video from sites with poor access to the Internet.

With the right USB modems, it was possible to add external antennas to get further range as described in the higher profile setup here:  

https://www.youtube.com/watch?v=xovEfYqNPG8&t=180s 

I seem to recall at the time you could get more generic 'cellmux' boxes to create ad-hoc Wi-Fi hotspots for similar reasons. Might be worth a look to see what there is these days.