What do you mean by link? To use them together. It is my understanding you cannot link two phones. You can link a phone to a tablet or computer, but not another phone. If you mean to link to transfer your apps and data over, you must use Smart Switch.
What methods have you tried to connect the two phones together for data transfer? Smart Switch is what you want to use to transfer everything from your old phone to your new phone and supports both wireless and wired connectivity. Your S21 should allow one of these methods to work even if it's got a problem. Another option is to use an SSD or a thumb drive with Smart Switch to record a backup of your S21 which you can then restore to your S24.
