@DocKen 

Even though the iPad 3rd gen is older hardware there shouldn't be.  My iPad mini 3 has connected just fine to a Deco network.  The first thing to check is to make sure the iPad hasn't been blocked. You can check this in the Deco app's device blacklist section.  If its not then I would do is Forget any saved networks on the ipad and try to connect again.  This way if a password was inadverting saved that is incorrect it can be ruled out as a factor.  Next enable guest network to test.  Start with just 2.4GHz and see if it connects.  Then disable 2.4 and enable 5GHz to see if it connect.  If both are successful then try enabling both bands and see if it connects.  Let us know how that goes and if you encounter any errors or mesages.