Can I use it across different subnets or over VPN?

No. Peer discovery uses UDP broadcast, which only reaches machines on the same network subnet. If two machines are on different subnets -- even if they can ping each other -- they will not discover each other.

Most VPN configurations place the VPN client on a separate virtual subnet, so machines on VPN typically cannot see local peers (and vice versa).

This is a deliberate design choice to keep the tool simple and zero-configuration.