There is no parings between the WiiMs and your phone. The phone will discover the WiiM devices each time you start the WiiM Home App, using the uPnP protocol.
This do however require that the phone and the WiiM device is on the same logical network.
A new install uses a peer to peer connection directly between the phone and the WiiM (over WiFi). This is only used once at setup, so if you see this every time, the setup didn't finish or the network setup was wrong.