>The WAN ports were configured with separate public addresses - a unique one for each router.
Interesting, so this sounds like a different configuration than some bonding setups which appear to require only a single public IP address. I wonder if that's down to how the ISP configures their interfaces.
I suspect I could make a lot more progress here if experimentation didn't mean scheduling connection downtime of an unknown duration, which in practice means being physically present in a locked office building in the dead of night (for which honestly they Do Not Pay Me Enoughâ„¢).