Thanks for your answer! I managed to get it to work with my Mango - turns out, the modem keeped working as a router itself when plugged in anywhere, so I had to disable its own automatic 4G connection and only control it via Mango.

However, the connection was never stable enough - it dropped (either 4G or Mango’s Wi-fi) every ~30mins, and the internet speed degraded dramatically.
Everything’s fast and stable when the modem works on its own though.
So I’ll try to send the router back now.