No mqtt discovery from remote zigbee2mqtt and mosquitto server on another Pi

I’ve got zigbee2mqtt and mosquitto installed on another Pi in the same LAN.
I know it’s available as I’m using Node Red in HA to subscribe to related zigbee device topics.
I have set both HA and zigbee2mqtt to enable HA. I can see the configs being published in the correct format in the remote mosquitto. I have set HA configuration.yaml to point to my remote mosquitto server using the following:
discovery: true

I don’t see any entities being picked up through the discovery mechanism.
Any help would be much appreciated.