Make sure you have created the SMS account and set the routing groups by following this article.
To apply the routing group as a Buyer of SMS Service, please follow the following steps:
1. Login as a Buyer > Account > Edit the SMS account you want to arrange.
2. Select the Routing Group on the "Default Routing Group" drop down.
By selecting the Routing Groups, your SMS traffic on that account will follow the Routing Groups' routing arrangement for all destination.
You can update your Routing Groups anytime you want, and it will be applied immediately.