Bandwidth Report for multiple WAN instances

The Interface Bandwidth Report is a special version of the Device Metric Trends Report, designed specifically to make it easy to get interface metrics.  While it is designed to meet requirements in most cases, it isn't going to work perfectly for everyone.  If you use the Device Metric Trends Report template instead, you'll have more control over what goes into the report - you'll be able to pick specific instances, and you can even use glob.  So if your WAN interfaces are named such that you can match them all with a glob expression, you could set up a Device Metric Trends report that, for example, matches all devices, the snmpIf- datasource, all instances that match *WAN*, and the specific interface datapoints you want to see.