Unless you are using the (allegedly awesome) RadioLib, most other stacks don’t cope well with the all zero JoinEUI - particularly not LMIC - and the Heltec stack is so old the other copy is in the Smithsonian.
If you want an official JoinEUI, create a device with all zeros on the JoinEUI, copy the DevEUI it creates for you and then promptly delete it that device so it doesn’t get in the way.
That then gives you a JoinEUI you can use for your devices and TTN will generate an official EUI for your device on creation (up to 50 per app). And it will generate your AppKey.
If you need to create local addresses, then this mildly awesome webpage does the job: Random EUI or Key generator