Is it necessary to do some configuration to save the hr? Tried it in swimming app, the HR as on the datafield but after save the activity it didn't appear.
No, the heart rate should be recorded and saved into the FIT file without any additional configuration once the data field is activated. Note that the ANT+ signal does not work underwater, so perhaps the watch was not able to detect the HRM in your case?
I can see the setting of the ANT ID but not the change in the name of the field, haven't checked to see if it's changed in GC/GCM.
BTW I found that the FR235 can indeed create a second ANT+ connection to an external HR, you just need to set the first external HR in the sensor settings and if there's another one around AHR will connect to it once the activity is launched. Great to troubleshoot my current strap/pod problems ;-)
No, I tested it in my living room at home :) Just moving the arms like i was swimming crawl, In the datafield it displayed the HR accurately, but in garmin connect i can't see the HR data. I have the fit file but i don't know how i can open it to see if ir recorded the HR.
Tried it again this time in a real swim session. The datafield worked fine and showed the HR data during the activity but in garmin connect there's nothing.
Activity here: https://connect.garmin.com/modern/activity/1594431715