Stuck connecting

[yajo]

I'm experimenting with this tool and, when I connect from machine B to A, in A this is how it stays:

$ shwim
┌─────────────┬───────────────────────────────────────────────────┐
│       ShWiM │ Instant terminal sharing via Magic Wormhole       │
│             │ Once connected, we launch tty-share               │
│             │ Note: --read-only was not specified!              │
│             │                                                   │
│  Magic Code │                  <code consumed>                  │
│             │                                                   │
│      Status │ ✅ Connected ws://relay.magic-wormhole.io:4000/v1 │
│             │ ◐  waiting for peer to use above Magic Code...    │
│             │                                                   ���
└─────────────┴───────────────────────────────────────────────────┘

And in B, it stays:

Connecting to peer

So what to do? What's happening? Do I have to open some firewall ports or something?