Finally, the issue was an unprovisioned gateway. My vendor forgot to unlock the device. I was able to detect that with the following POST request (substitute with your gateway id):
$ curl -k -i -X POST --data '{ "router": "xx-xx-xx-FF-FE-xx-xx-xx" }' https://mh.sm.tc:7007/update-info HTTP/1.1 404 Not Found Content-Type: text/plain; charset=utf-8 Content-Length: 15 Date: Sat, 02 May 2020 21:13:07 GMT Server: Python/3.6 aiohttp/3.5.4 Connection: close Not Provisioned
Especially thanks to @KrishnaIyerEaswaran2 who assisted even on a Sunday!