Doorbell won’t connect to hub

So this is my dads old system as he has upgraded but I have the 2k wireless bell and hub the hub connects absolutely fine and I get connection but when I try to sync the doorbell it says it isn’t found, has anyone else had the same problem, it has been connected to 2.4ghz so it’s not that and I’ve also tried resetting the doorbell but still doesn’t connect
Many thanks