ISP IP trunk

Hello,

I have CCR2004-1g-12S+2XS to SFP+ 1 port connected ISP provider with 5 External IP address.
I need trunk one external IP address to SFP+ Port12. In this port customer connect router and configure on its external IP address. How i can this do ?
ISP gives me static IP addresses. e.g. 85.222.142.28/23