If Santander said "passive" IP address then as far as I know there's no such thing.
If they said or meant static IP address then they must be mistaken. The vast majority of domestic internet users have dynamic IP addresses. When you access your ISP it allocates you an IP address. If you unplug the cable of the modem and plug it in again you'll get assigned an IP address but there's no gurarantee that it will be the same one as before - hence your IP on the web is called dynamic because it changes every time you disconnect and re-connect. If the Santander website didn't work with dynamic IP addresses then no one on bog-standard consumer Virgin, or any other ISP, would be able to access Santander.
Some people and a many businesses need to have the same IP address all the time; this is called a static IP address and it's a service from your ISP that have to pay extra for.
The only thing I can think of is that you might be using a VPN. If so then you probably need to disable it because many banks don't accept connections from VPNs. If you're not using a VPN, what exactly happens when you try to log in to the Santander site?
