You can’t record from an external input on any true pvr. They can only record from the built in tuners. Briefly digital tuners produce a string of zeroes and ones which is simply copied to a hard disk. A DVD recorder is capable of recording from a box with analogue output. Not familiar with a Now TV box, the fire stick though only has hdmi digital out which has HDCP copy protection.
There are no audio or video inputs on a Humax Freeview+ Youview, FreeviewPlay or Freesat+ box. Only outputs.