CH21 is not a Freeview frequency. DN channels are 28, 32, 34, 36,and 38. See here
Ch 21 sounds like a normal analogue modulator frequency and your old analogue TV is tuned to it. Presumably your STB is connected via this modulator? If your Samsung TV is able to be set to NZ and 8mhz then there is no reason it can't receive signals unless there is no proper UHF antenna connection. Try connecting your antenna cable direct to the TV and manually tune the above channels (28 - 38).
Would something like a band pass filter help at all?
Usually it takes away the interference from other signals in the network and supplies the digital television channels to your TV without an issue.
OK. Quality may be 0 but what about signal strength?
Also probably a stupid question but have you plugged the UHF antenna into the Ant socket and not the Sat Dish Socket. According to the manual yours is a dual tuner (Sat & UHF).
The "signal quality" is at 0 for each channel selected.