Turn the wifi off on the SH2, connect a router of your choice to it in access point mode. Make whatever use of your own router you wish - vlans, separate wifi networks, whatever it can offer.
DDNS can be set by any device on your network, it doesn't have to be the first thing connected to your wall.
Only BT could make such a mess of digital voice implementation.
Provide a DECT telephone adapter that people can connect anywhere to their network rather than tie them to your smarthub.