Multiple WAN ip subnets / addresses

Question

Good day, 
 We purchased a new subnet for a client of us. 
 The default IP is like: 
 
 217.63.AAA.BBB connected to Port2 
 
 we have a subnet with the ip: 
 212.115.CCC.DDD/29 
 This ip range is routed to 217.63.AAA.BBB 
 
 I wanted to add an alias to the Port2 with the subnet 217.63.AAA.BBB/29. 
 The error I receive is: You cannot enter a network IP for IP Address 
 
 I cannot understand why this is. With all other advanced routers it is so simple to configure. 
 What am I doing wrong? 
 
 thanks in advance, 
 regards Patrick

Patrick Wille · Accepted Answer

Had to add the specific IP with subnet 255.255.255.0. 
 Id doesnt add the complete subnet at once.