First conversation with them afterwards was how to configure the supplied router with more than 1 IP address. It would seem this cannot be done (surprise!). So with a collection of Cisco equipment and a BT VDSL modem at hand, I went to configure as replacement config. Then I checked and found while I had details of four IP addresses there were no details on the gateway address required.
Second, third and fourth calls/online chats resulted in the following:-
You have reached TalkTalk Business support, how may I help you?
Me (16/05/2019, 12:12:40): Updating the firewall and need the default gateway address, it isn't shown on the supplied SuperRouter and this is being replaced
TalkTalk Business Live Agent (16/05/2019, 12:15:34): Default gateway is 192.168.1.1
Me (16/05/2019, 12:16:00): I doubt it, that's a private IP address
TalkTalk Business Live Agent (16/05/2019, 12:17:04): That's the gateway.
Me (16/05/2019, 12:18:42): really. So if I have an external P address of 184.108.40.206 the gateway address is 192.168.1.1... Can I talk to someone who understands IP addressing and routing, please
TalkTalk Business Live Agent (16/05/2019, 12:19:23): The address you may be referring to is not one we can access, we can only suggest you set the router to auto or dynamic and it will get the address.
TalkTalk Business Live Agent (16/05/2019, 12:20:06): You could try calling 08000833003 if you wish, or chat again and you may get someone else.
Me (16/05/2019, 12:48:20): the last guy suggested an external gateway address of 192.168.1.1 ! this is getting silly for a business account
TalkTalk Business Live Agent (16/05/2019, 12:48:49): That is to direct you to the login page for a TalkTalk Business router.
TalkTalk Business Live Agent (16/05/2019, 12:49:13): Which would not apply in your case as you are switching to a BT VDSL modem.
TalkTalk Business Live Agent (16/05/2019, 12:49:58): Our 2nd Technical Support Team are not customer facing.
Me (16/05/2019, 12:49:59): Yes, I know and the TalkTalk routers don't allow you to access additional fixed IP addresses, only a single one.
TalkTalk Business Live Agent (16/05/2019, 12:51:07): I am sorry the information I have provided is not sufficient enough however this is the information I have found when looking in to your query.
Me (16/05/2019, 12:52:12): OK, so basically I'm being charged for something I can not ever use ! i.e. more than one fixed IP address
TalkTalk Business Live Agent (16/05/2019, 12:54:07): If the router is third party, we can only offer limited assistance as we do not support them.
Me (16/05/2019, 12:55:04): So, as I said, the supplied router/modem won't support it, and you will not supply even the basic network info to use a 3rd party one.
TalkTalk Business Live Agent (16/05/2019, 12:56:18): As your network provider, we are responsible for providing you broadband connection to the premises. The configuration of your router, if 3rd party, is not something we have information on. As I have said, The details you require should automatically configure themselves so long as the broadband credentials are correct.
Me (16/05/2019, 12:57:07): but without a gateway address the router can not send the credentials anywhere
TalkTalk Business Live Agent (16/05/2019, 12:59:01): We do not have this information. I am sorry about that.
Now two things here, they happily sell you 4 IP addresses (only 2 of which you can use), but more to the point the basic concept of
static IP address -> gateway address -> rest of the world
seems beyond their support!