Hi … the requirement of 2 fixed IP’s is due to the equipment you are using. You should be able to use a number of devices on the on IP if you have a router with prot forwarding and it will work the same.
The solution is that we use just a plain adsl modem which has ethernet port and connect it to the ethernet interface of the A800. But then we will not be able to place anything else of a data network. This should work fine.