The comment by Andreas Weber is not quite correct, the router must be in the network but not necessarily in the middle. Antennas can also communicate with each other according to product specifications, https://amplifi.com/docs... see page 5.
In a mesh system, every point can theoretically connect to every point and also bridge failures in this way. Where the network access point is located is irrelevant as long as it can be reached via the mesh points.