Your other posts asks about recommendations for phones and SIM cards that are hard to track for LE (Law Enforcement). This post gives a bit more detail about your day to day use case. I think I know what this "job" really is.
As you probably realized by the replies you've already gotten... cellular networks are NOT the way to go. If they operate in the US, even if just partnering with a US network but owned by a foreign entity... they comply with US law.
The Mobile router is only going to add an extra step, as the cell provider will triangulate that device.
Once LE has an IP, it's usually just procedural to get a good precise location. The best protection is to use layers of encryption and a messaging app that will obscure your IP. But even then, if LE can start with a previous location, they often don't have to get an IP from the app. (e.g. previous places of interests, list of all devices in the area, and see if any matches that show up at the next place).
I've looked into creating larger WiFi footprints using high gain antenna and drones perched high with solar panels, but it gets very expensive. The idea is to create some distance (acres of search area) that separates the known precise location of the cellular modem, and the actual person. The only real way to evade, is to own and operate your own network as much as possible.
This technique still only adds an extra step, as LE would only need a local van that can just track clients of your WiFi network once they pinpointed the cellular device as the same as the WiFi AP.
Maybe cellular and WiFi are not a good fit.
Look into radio systems with Frequency Hopping, burst and encryption. Very, Very expensive... but really the hardest to track.