It sounds to me like you shouldn't have configured a multi-WAN failover, but I don't know if it's even relevant to what you're asking here.
If i understand this setup correct, your WAN address is 66.x.x.x. That means you need to add 66.y.y.y as a virtual IP, and then use 1:1 NAT for that IP to 172.16.x.x. Once you set that up, you just need to set your firewall rules to allow access to the ports you want open.
@FSPL:
Also, once the inbound traffic can access that IP, is it possible to have them exit the same way they came in?
I don't know what you mean by this exactly. If you want connections that are initiated by 172.16.x.x then you need to set that up in Outbound NAT using AON. You should already be using AON to use the RR connection for web browsing, but it's hard to tell from your diagram because LAN is missing.