It would be up to Amazon to create such an app, as Fetch is only based in Australia and their equipment running its own software - the market is quite small.
Its available on other platforms such as Roku (Telstra TV) as Roku are a large international brand and thus have a huge user base.
Was one article I remember earlier in the year saying that Amazon Prime Video was comming to Fetch but nothing has happened with that.
Just saw a Fetch TV advertisement on TV and guess what? Prime Video was displayed on it. I contacted Fetch TV and they were surprised on the ad, which I saw on channel 94 Life. Interesting that I haven't seen it since.......
Anyway, I was informed that around April 2020 Prime Video should be on the Mighty box, and hopefully on the Mini not too long after.
Just FYI, looks like we will have Prime sometime soon. I hope.