Do you own your public IPs and advertise them to the ISPs or are the public IPs ones that the ISPs allocated to you and you don't advertise any public IPs to either ISP.
I suspect it is the second one ie. you were allocated them and if so then what you are trying to do won't work as far as I know.
The issue is the public IP you are trying to use was allocated to you by the backup ISP. They own this IP and are responsible for advertising it to the rest of the internet (as part of a larger range).
The primary provider does not own this IP ie. it is not part of their public IP address space so they don't advertise it to the internet.
This means all traffic to that public IP will come via the backup ISP.
The primary provider won't advertise out a single public IP that they don't own for you so you really only have a couple of choices -
1) use static PAT for the server if you only need access from the internet on certain ports. I'm assuming you have at least one public IP from the primary provider allocated to an interface on a router/firewall that you administer
2) if you can't use static PAT then you need to purchase some more public IPs from your primary provider.
There are no other solutions I am aware of.
Jon
