Note that 'adaptor' was forced on me as the system doesn't want to know about the real model number

I have a brand new old stock TD-8840T modem router which I wish to use on a network with both private and public IPs. the unit is a replacement for an old Netcomm NB6 which died a week ago. Before anyone starts on about ADSL being past its use-by date, Australia is a backward country, especially away from the capital cities, and ADSL will be the only practical internet connection for some time.

Both the old Netcomm NB6 and the new TP-LINK TD-8840T have a function to add a second IP range, and this always worked perfectly in the Netcomm unit,  allowing internet access to all LAN machines with a public IP address.  For various reasons, VPN isn't relevant to these machines.

When I configure the TP-LINK TD-8840T exactly the same as the old Netcomm NB6, I can address all machines with public IP addresses from LAN machines with private IP addresses, but I attempts to address the LAN machines with public IP addresses from the internet fail (timeout). It doesn't make the slightest difference whether the primary IP range is private and the secondary range public, or vice versa.

Is there something in the setup that I've missed ?