How to add a remote device, enable sync on CLI? Says: rejected: unknown device

On my desktop computer, I have tried to use Syncthing WebUI to add my remote headless server (no GUI/display, only CLI), which was established by just running ./syncthing command. I have got the ID from the CLI output.

Yet my WebUI tells me "connection refused" and "context deadline exceeded" and the headless server output shows "rejected: unknown device" next to my connection attempt.

In a "syncthing --help" output, I am unable to find any command which would add mine defined remote machine ID and allow its connections.

I have tried to access http://headless server IP:8384/ but it timeout. I am unable to find any firewall blocking, another service on a different port is accessible.

Any idea how to proceed interconnecting the two devices simplest way?