Router with Firewall

Hello,

we are setting our branch office

we were using DIA so we get cisco 881 router from the ISP

no we will get fortinet and I know how to work in it

but I know usually the firewall will be at the edge of my network

how will be the design with the router ? will I do the nat in the router side and the firewall policy in the fortinet ? or should I connect only the fortinet and do the nat , firewall "which is VPN site to site , FTP, Web filtering"

for example if i have 4 public IP

88.88.88.88-88.88.88.92

so will I do nat in the router for these public IPs to for example the 10.1.1.1 and then open port 25 in the fortinet ?

and will VPN site to site will work if i assign private IP to the fortinet and nat it in the router ?

Please advise I know the configuration but I need the design of connection as best practice