Laser mesh...
There is an ongoing fight between the UK radio regulator and illegal radio stations. The regulator has detector vans, so it's a similar situation to what you describe.
They can find transmittors, but their objective is to find the studio, which needs a live link to the transmitter.
As I understand it, the pirates break out onto the roof of tall buildings, especially public housing towers, and place transmitters. These will be found, so they gotta have a way of linking them back to home.
Sure, they could use mobile data (has its own opsec problems), or hacked residential wifi and a VPN (add real prisong itme of a hacking charge to the sheet when you get court).
Apparently what they often do is use a cantena link - a focussed beam of wifi to some distant place, which can be very difficult to pinpoint accurately. I also saw a doc where they used lasers.
These days though, I think a lot of them just stream on the internet.