If the manufacturer sells this with their own network, and does not officially support other configurations, then beware:
If you register this on TTN, then you need to make sure that only TTN gateways receive the join request, or that the device is no longer registered on any other network. If the vendor’s gateways also receive the OTAA Join Request, then both networks might transmit a Join Accept. When that happens, the device might handle none, or might handle the wrong one.
The name “JoinEUI” suggests this is a 1.1 device. That should be able to connect to a 1.0.2 server just fine (using the JoinEUI for the AppEUI), assuming its firmware is compliant (to recognize the 1.0.2 response), assuming it does not require specific 1.1 functionality, and assuming it only needs Class A. The TTN Community network is using 1.0.2 and only supports Class A.
If the device needs specific 1.1 functionality, then you’ll need to run your own TTN V3 stack (or use TTI’s commercial offering). But as a 1.1 JoinEUI should/might have been registered in the LoRa Alliance’s
joineuis.lora-alliance.org DNS, you might also need to make them change that entry.
Also, as soon as the TTN Community network supports 1.1 in the future, a new OTAA join will use the 1.1 join procedure. I’d assume that if TTN knows the JoinEUI in its own network that it will not use the join server DNS resolution. But if it does, then this might suddenly throw you back to the manufacturer’s join server?