Public ip behind modem/router - setup

Question

I have been reading this forum's topics thoroughly over the past few days - very helpful advice from some of the top posters.
However, I have a very basic question which is beginning to make me tear my hair out - how do I give the external NIC a public IP address that's completely visible over the internet but that will still allow communication with my router/modem.
Last week we decided to update our old G4 running OS X Server 10.3.8 to a brand new XServe. Our needs are very basic and the old server ran afp, nat, mail, firewall and vpn for a small workgroup of about 10 designers. Our ISP gave us a range of static public IP addresses of which we only used one for both our router address and mail (over smtp). ISP handles external DNS and sorted the MX records for this. A zoom modem/router was configured in half bridge mode to transparently send this IP address 83.xxx.89 to the OS X server via DHCP. Therefore external NIC was set up as DHCP and internal NIC was set up with local address 192.168.0.1. Because the 89 external address was also our mailgate address - mailgate.mydomain.com - our ISP could send down our mail, mobile users could log on via VPN, workers on the lan could browse the web and use afp etc. In other words, if worked perfectly for our small setup and was steady as a rock for about 3-4 years. I have noted comments about the lack of a hardware firewall - we are relying totally on Apple's firewall in OSX server but I am comfortable with this for my small workgroup.
When we came to install the XServe in a similar fashion the external NIC would only pick up the external public IP address for about two minutes. After this it would drop and pick up a self assigned IP address - obviously all connection to the internet would be lost and the primary address would become 192.168.0.1 because the internal NIC would move up in rank. After talking to Apple support for a literally a couple of hours they seemed to draw a blank but suggested that I must change the external NIC to manual to stop it being overridden. I must also set up LOM correctly. Fair enough, I'm willing to try this and have now got my ISP to send our mail down to a different mailgate - 83.xxx.90. Therefore we have one public IP for the router and another for the mailserver (that happens to be doing a few other services too). Domain names for these are mailgate.mydomain.com and mailgate2.mydomain.com - in fact the original address will still act as a mailserver with a slightly higher priority because I want the option of switching back to my G4 until all this is resolved and don't want two new setups on my hands! Once the Xserve is setup the old server will no longer function.
However, all the modems I have tried will only communicate with internal IP addresses on the LAN. Even the dmz instructions say I should configure with an internal IP address - is this correct? Turning off DHCP server and NAT on these devices doesn't seem to help either (I assumed this would be the answer but no). Can someone tell me where I am going wrong? I can't find anyone else asking about this so I can only assume that I have overlooked something really simple.
Ideally I would like to keep the new two address setup but I am quite happy to go back to the single setup with the router in half bridge mode if the Xserve will hang on to it reliably.
My set up is:
Router:
WAN static 83.xxx.89 subnet 255.255.255.248
LAN 192.168.1.1 subnet 255.255.255.0 (don't really understand where this fits in).
NAT off, DCHP off
Xserve
external NIC
WAN static 83.xxx.90 subnet 255.255.255.248, router 83.xxx.89 or 192.168.1.1 (tried both and tried matching both subnets)
internal NIC
LAN 192.168.0.1 subnet 255.255.255.0
LOM (guessing here a bit because manual is awful)
WAN static 83.xxx.91, subnet 255.255.255.248 (channel 1)
LAN 192.168.0.50, subnet 255.255.255.0
DNS on XServe will be setup up so that mailgate2.mydomain.com resolves to the internal 192.168.0.1 address. External DNS handled by ISP. Reading previous posts this seems correct.
Router/modems are Belkin F5D9630uk4 (hopeless, support tell me it won't take a static WAN address so it's going back), Voyager 205 (seems quite configurable but won't let anything through if NAT is disabled), Zoom X5 (older 4 year model currently working succesfully in half bridge mode for my older setup - however only new models will work in full bridge mode which could be what I am looking for??)
The realise the above modems/routers are more consumer models so I am willing to buy a more configurable expensive pro modem or router if this is what's needed. I stress though that I would like the Xserve to be acting as the firewall and NAT etc and don't really fancy having to have to forward loads of ports etc.
Thanks for taking the time to read this through, I thought more detail would be helpful. I am also prepared to employ a professional to set this up for me but they (Apple knowledgeable) are rather thin on the ground in Glos UK. We did get a company in four years ago to set up the old server and they gave up after three weeks (a friend and I managed to crack it the following weekend - probably just lucky!).
Any help would be really appreciated - what we're trying to do must be pretty common?
17 G4 Laptop   Mac OS X (10.4.8)  

Answer 1

Now solved - bought myself a more professional router that has more user features and all has become clear. Router is Draytek 2800 which seems like a good piece of kit. I think that there are various ways of achieving what I wanted to do (I suspected this all along) but I chose to set up a 'second public lan subnet' in Draytek speak. See here for more:
http://www.draytek.co.uk/support/kbvigor2ndsubnet.html
Thanks to those that posted.