The problem is that Garmin Connect forgets the fr245 about every week. To be clear: the problem is not bluetooth disconnection. His Garmin Connect looks like he doesn't have any garmin devices and we need to add and pair the same fr245 again and again every couple of days.
Background: I gave my fr245 to my son (N) 2 years ago. He used it without any problem with his old Samsung phone. A month ago I bought a new phone for me so I gave him my old OnePlus phone. Previously I used this OnePlus phone without any problems with my Garmin watch (and in the far past even used it with the same fr245 that is my son's now). The Samsung phone went to my other son (M). We did a factory reset on both the Samsung and the OnePlus phone (so all bluetooth pairing was removed from both phones) and we paired the fr245 to his "new" OnePlus phone.
