Got it!
For reference,
I had to set /etc/rabbitmq/rabbitmq.config to look like:
[
{mnesia, [{dump_log_write_threshold, 1000}]},
{kernel, [{inet_dist_listen_min, 9100},{inet_dist_listen_max, 9105}]}
].
& unfirewall 9100-9105 on each server for each other.