I am considering buying a BCD536HP when the siren app becomes available. Will the siren app allow me to connect to the BCD536HP with my android cell phone when I am in my car and there is no router available?
If you are asking if your cell phone can connect with your BCD536HP while it is at home and you are in your car, yes you will be able to.
The only caveats are that your scanner at home has interent connectivity as well as your cell phone having internet connectivity, wherever it happens to be.
On the other hand, if both you and your scanner are in the car and your cell phone will be acting as a "remote head", you can do that as well. The "dongle" is what your cellphone uses to "marry-up" with the scanner.