For this case you would need to do the following.
Routing
Create a secondary default route that points to the RF interface on the SSG and has a higher metric/preference than your normal default route.
Create a track ip monitor for your internet facing interface so it will be automatically taken down when the internet is not reachable. I typically use the two carrier DNS servers as the ip addresses to ping to confirm internet access.
NAT
On the remote site that will access the traffic on the RF link you need to add a nat rule for the new subnet going out to the internet. This will be a policy from the remote address to any untrust with nat interface enabled.
I will assume with the link in placy you already have routing and policies to allow the two internal subnets to communicate.