Can't connect to RDS with ConfigServer Security and Firewall enabled

When I try to connect to my RDS instance it fails with a (111) error. If I disable the firewall:

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csf -x

I can connect. If I try to add the instance via endpoint:

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csf -a <RDS Endpoint>
csf errors with:

Code: [Select]

<RDS Instance> is not a valid IP/CIDR

Is there a way to make this work without having to disable csf?