Correct me if I’m wrong but when you open a connection using an xDSL or ISDN modem, you see a new interface at TDI level.
But this interface is managed by NDISWAN which makes the translation from Ethernet packets to ppp. So you don’t see any new adapter which is bound to the TCPIP protocol when you open this kind of connection as the packets go to NDISWAN which then transfer the packets to the good miniport.
So the only adapter opened by TCP/IP when you use multiple WAN connections is NDISWAN and there is no way in NDIS layer to associate the adapters intercepted in NdisOpenAdapter to the interfaces at TDI level.