[Help] Can’t get sftp client to work behind firewall appliance.

Hello. I manage a small network that is required to have an external firewall appliance. Unfortunately, one of the computers on the network is required to send information to an sftp server. There appears to be something keeping the traffic from passing between the firewall appliance and the ISP’s modem. Not sure if it matters, but those are a Watchguard T15 and Windstream T3200. If anyone has any suggestions, I’d appreciate it. I cannot see that either is blocking the traffic directly, and I cannot bridge the modem.