You'll need to do two things. First, on the WAN page make sure you uncheck the box that says 'Block private addresses'. Second, you'll have to make a firewall rule to allow traffic to pass through.
You'll want your rule on the WAN interface something like this:
| Protocol | Source | Port | Destination | Port | Description |
| Any | 10.10.10.0/24 | * | 192.168.100.0/24 | * | Description |