Hi Rich,
I had the same problem with avanta plus 39c and found the answer on the netatmo forum at the following link:
https://forum.netatmo.com/viewtopic.php?f=19&t=5959&p=28373&hilit=avanta+plus#p28373
Ensure that terminal block X9 and X2 are as per the picture (see below)
Would love to...