Check what the device reports on OIDs ifSpeed 126.96.36.199.188.8.131.52.1.5, ifHighSpeed 184.108.40.206.220.127.116.11.1.1.15. We use these OIDs when polling Interface Speed.
You can make these changes in bulk. Go to the Settings > Manage Nodes page. Then change the drop down from Nodes to Interfaces. Do a multi-select on the interfaces for which you want to change speed, 'Edit Properties' and set the custom bandwidth.