Hi Joe
Thanks again for your help! I soldered a short 50 ohm cable to the RFM69HW with an SMA connector at the other end in order to be easy to test with different antennas. I have manageded to make it work with a 1/4 wave antenna (simple wire) as far away as possible from the RFM69 module (I have it now glued to the bottom of the enclosure). If I put it a bit above that, it only transmitted if I put my finger over the antenna wire (that is isolated), but it received well with or without the finger on the antenna wire. I have replaced the RFM69 module, thinking that it could have some problem, but the result was the same. I thought if it could be some interference of the transmitted signal on the wires between the ESP8266 and the RFM69, but before assembling in a protoboard it was tested in a breadboard, with much longer wires. I tested it also without the antenna connected (just the cable with SMA connector) and it transmitted always, but of course with less signal.
So in summary, it is working, but the antenna placement can cause the node to stop transmitting, and the node receives well no mater where I place the antenna (with different signal levels but always receiving). My main worry is the stability, since I can't identify a reason for this problem. I have used many RFM69HW modules in the past and never had a similar problem - it is true that it is the first time I'm placing the antenna inside the enclosure, in every node I have, I used an external SMA antenna or the simple 1/4 wave straight wire going outside of the enclosure.
Best regards
Fernando