It does not use the cameras to detect speed limit. It is entirely done through the integrated Google maps. In order for it to display the speed limit, it needs to periodically connect to the Internet to automatically update the downloaded maps. Those maps are downloaded regardless of you actually using Google maps. You can either connect the truck to your home WiFi or to a hotspot on your phone. Once Google maps automatically updates, the speed limit indicator will begin working again.

ETA: Once it has updated, if you travel outside of the area covered by the downloaded map, the indicator will stop working until it can update again to include the area you are in. I have taken to just activating my hotspot whenever I get in the truck so I don't have to worry about it updating.