Issues with two devices unable to establish a two-way tunnel behind firewall. Cisco FMC (yay)

Kinda stumped on an issue right now with two devices and our firewall.

I have two devices that need to reach out and create a connection to a remote server. Apparently, they will be creating a tunnel between themselves.

I have created access rules to allow these internal devices outside which seems to work, but no connection comes back in. I created another access rule to allow these outside IPs access inside. Still no connection.

When I run a packet tracer (cisco FMC) I am getting an error related to NAT.

There is a general NAT/pat rule for user/server traffic outbound to the outside interface within this IP space.

I haven't run into an issue like this before so I am kind of stumped as to where to start. I am assuming somewhere in NAT but packet tracer isn't always the most reliable.