Can't stream Amazon rental movies on ROKU 3 since April

For about three months I haven't been able to load any movie I rent from Amazon. I can load anything I own outright in the Amazon library. Amazon says AT&T tells them it's a problem only with certain devices inc. the Roku.  This makes no sense, as I can stream certain Prime Amazon TV series every night without issue, in full 1080p. I do all streaming from my Roku 3, which is renowned for its processing speed among streaming devices.  I was given a list of steps to follow, according to what AT&T told Amazon would resolve the issue, but I don't know know how to "disable the DNS cacheing".   Can you please help?