It is working now… I added a “new” antenna to the motionsensor and drilled a 2mm hole for it in the back of the letterbox.
Now when motion is detected I get a notification on Google Home.
Great. 
Just a note that this very much depends on the circuit used. Yes, a resonant antenna is normally better, but you will normally (is always!) find that the antenna used is matched to the circuit - there will normally be a pi matching network (or something else) that provides matching for the antenna to the output of the transmitter to avoid high VSWR. It may work ok with a different antenna, or it may not 
Thank you - so I am lucky - it is working… 