Does anyone happen to have Draytek's Multi-NAT (multiple Public IP's) working with the Vigor 2962?

My FTTC provider provides me a small range of public IP's, and I've had these working perfectly for many years with Multi-NAT on a 2862. I use static private IP's on a couple of LAN hosts and then the DMZ host feature to map the public IP's to those hosts.

I've replaced the 2862 with a 2962 in preparation for an FTTP connection due in a couple of weeks. I am planning to load balance the old and new connections. I've bought a Vigor 130 modem and attached that to WAN 2 on the new 2962, and connectivity works fine. Apart from Multi-NAT. I just can't get the WAN alias addresses to work, and think I've tried everything. No matter what I try, the static IP LAN hosts, that should be mapped to public IP's via the DMZ host feature, just route out as the default WAN IP

I've even managed to rule out the 130 modem, by going back and setting that modem up on WAN 2 of the 2862, and with the usual settings, Multi-NAT Public IP's work perfectly.

The 2962 is on latest firmware (3.9.6.1) and manually setup from scratch rather than any attempt to transfer config from the 2862...

Thanks in advance for any tips!