Skimming the documentation (thanks!) shows the device supports OTAA. But the only way to configure it is by using downlinks?
I’d assume it’s configured for OTAA by default, as then you could connect to any network, and then fine-tune it using downlinks. Do you have access to a gateway log? (Like in TTN Console, the gateway’s Traffic page?) If yes, then I wonder if you see an OTAA Join Request there. That would reveal the DevEUI and AppEUI, but not the AppKey, so still doesn’t help you much, but tells you it’s using OTAA. But if you see an uplink with the known DevAddr, then you know ABP is used (and ignored by TTN).
If OTAA is enabled by default, you need more details from the manufacturer.
If ABP is enabled by default then you’re out of luck with TTN, if only as the ABP and OTAA configuration is marked as RO, being read-only. Some other networks (not TTN) might allow any DevAddr though, even though that’s really not how things should work. (Like apparently in September 2018 both Loriot and Blink Services accepted a DevAddr that was actually assigned to the KPN network.) You could then use a downlink to reveal the LoRaWAN MAC configuration (DevEUI, AppEUI and AppKey) and after that configure it to switch to OTAA, and use it on TTN.