SolusVM module not connecting

First of all I have to say that I am very new to WHMCS.

I've successfully installed WHMCS running with php 5.4 and all is working fine.

I need to use it to connect to a SolusVM system to create VPS systems for customers.

I have successfully installed the SolusVM module into WHMCS and all looks fine.

Next I come to configure the SolusVM and WHMCS systems.

1. I have created an API User is Solus
2. Added a server in WHMCS using the details from the API users (e.g. IP, API key and ID, etc)

Now when I come to create a product and and go into the "Module Settings" screen. I choose solusvmpro as the module name and after saving that I understand I should be able to get Default Node, Default Plan, and Default O/S populated from the SolusVM master. There is nothing in these fields.

If I come out of the product screen and go back into it it can take about 5-10 minutes to get into the screen. This makes me think it's trying to communicate with the SolusVM master but not getting any response.

I have enabled module debugging in WHMCS to see if they sheds any light. Nothing in the debug log.

I have checked the API log on the SolsuVM end. Nothing in that log.

I use a Draytek 3900 firewall in front of the SolusVM systems (but there is no NATing involved, the VM master has an external address.)

I have check the firewall logs to see if anything is getting blocked. I see no traffic at all from WHMCS to the VM master (no traffic at all to the VM master in fact).

Can anyone suggest a good way to debug this please? Can I set enhanced debugging of some sort in WHMCS to check exactly what it is doing?