I assume, when you say "my ISP changes DNS" you really mean "my ISP changes IP address that DHCP issues". Changing "DNS" (that is, IP address of DNS server) would go completely unnoticed by the adapter.
does it ever happens "out of the blue"? In other words, if the router is up and running, and OBi is registered - IP address would suddenly change?
I can imagine you might have problems until the next registration (your adapter registered on the server with one IP address, and now it has another one). But as I said, I've never seen it happening... And if it happens after whatever brief loss of connectivity, adapter will register with the server with new IP...