I agree with @PhilipDAth on perhaps just leveraging an outdoor AP with a patch antenna for example, however I also have seen transportation customers with the setup @BrechtSchamp described, with a dedicated SSID that the mobile vehicle(s) connect to when they are within range. In some cases, I've seen a separate dashboard network with dedicated APs for that use case, so they can be separate from the management of the rest of the network at each site, but also managed as a single multi-site network for their vehicle connectivity use case.
I also have had K12 customers who had something like a technology bus that traveled around between the schools, and they also had wired devices on board the bus. So we used a Z3 along with a nano router like a TP-Link for example running in client mode and ran the Ethernet port to the WAN/Internet port on the Z3 and that worked fine. As the bus drives into each school parking lot, the nano router becomes a client on the school's wifi as soon as it's in range and provides the wired WAN/Internet handoff to the Z3 and all the wired and wireless devices have their access. Perhaps such a device could be configured with multiple SSIDs for each of your networks at different sites and run into the wired port of an AP through a power injector.
