Reading GPS coordinates on DB410c - GNU/Linux Debian and gpsmon



I know this topìc as already been threated here but I am having little problems reading GPS coordinates from the onboard WGR7640 RF receiver.

The Debian image installed is the 252 img (installed last week, now is labeled 260), but I think that the GPS patches and implementations have been already included into 252 version.

After reading the official instructions here at section Using the onboard GPS, I installed the suggested packages, I can see /dev/ttyGPS0 but running gpsmon just gives this output:

tcp://localhost:2947          JSON slave driver>
(84) {"class":"VERSION","release":"3.16","rev":"3.16-4","proto_major":3,"proto_minor":11}
(111) {"class":"DEVICES","devices":[{"class":"DEVICE","path":"/dev/ttyGPS0","activated":"2017-08-21T10:20:05.680Z"}]}
(122) {"class":"WATCH","enable":true,"json":false,"nmea":false,"raw":2,"scaled":false,"timing":false,"split24":false,"pps":true}

without any additional information.

Do I need to edit / apply some configuration file before reading from the on-board GPS receiver ?
Thank you in advance for your help.