I have the dongle installed and recognized in family hub fridge as well as smart things. I have a (tuya) zigbee contact sensor that the fridge will not pair. Tried many times. Reset sensor. Power cycle fridge.etc. Ironically it pairs just fine with my (old) smart things hub. It also pairs fine with Sonoff dongle on home assistant. So definitely the family hub is the issue. What do I have to do to get this paired?
Well as luck would have it, in an act of desperation because Samsung chat support was less than useless and I was getting so frustrated, while chatting with them I decided to try repairing yet again for like the 100th time but this time while the phone was searching I opened the fridge door and unplugged the dongle and replug the dongle back into the USB and then the sensor magically showed up this time so I don't know what's wrong but at least it paired now so maybe this helps somebody else in the future
For those following along. This is the end goal. If our lockbox is opened, it takes a photo from ring doorbell and sends us notification (also pops up on family hub fridge screen). Niiiice. I had to use the fridge hub because of the proximity to the metal box outside. The samsung white box hub in my office would lose connection to sensor when the lid closed. However I did the same on that one with a zigbee vibration sensor for my mailbox at the end of the driveway (which has the opposite problem as the office is closer and fridge has too many walls in the way) 
