NetFlow feature is available on AOS data products running firmware version 16.1 or later. But I am not sure if Adtran 1238POE supports NetFlow export. You can verify if it does from this link:
If it does, you need to enable NetFlow on the Adtran and have it send flow data to your SolarWinds NTA. Adtran terms their NetFlow as Integrated Traffic Monitoring and you can find some config information in the following links.
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