It sounds like you're tagging the SSID to be the same VLAN (999) as the native vlan on the switch (also 999). Is that correct? If that's the case then you would want to have VLAN tagging disabled on the SSID. By having it on both will cause problems. You would only want to set a VLAN tag on the SSID if you are wanting the client to have a different VLAN than the native vlan.
If I've misunderstood then some other questions. Are other devices able to join the SSID? Do those devices get the correct IP address? Is it only the Tesla that has a problem with the SSID? Any problems reported in Wireless Health?