When you are talking to Clover, it would be as well to let them know if they don't already, that Virgin Business and Virgin Media their residential service use different modem/routers, and this might just have an influence on the problem.
As this is the Virgin Media forum, you won't get any official support here for business uses.
I believe it has to do with the way the Hitron router makes a connection to VM over a GRE tunnel, and not allowing a proper modem only mode without having a group of static IPs... but without testing I cant say for sure.
I'm afraid there isn't a business forum.
Link to the VM Business contact details
I have now received a response from Virgin Business pointing me to Port Forwarding on the Hitron Router..... Clover use Port 443.
Not familiar yet with this but will investigate. This reply to give you an update in case you are familiar with the technique.