Orbi RBR50 firmware update problems

I got an email about a firmware update to v2.3.5.30. I'm currently running v2.3.1.60, and via the app I clicked on "Check for Updates" as instructed. The app ran for c.15 minutes and seemed to be hung up. So I closed it and reopened it, then clicked again on "Check for Updates." Got a message saying "There was a problem checking firmware update." So I logged into my account from my desktop and navigated to the page to download the firmware update. The img file was downloaded, but when I try to run it, I get the error "The following disk images couldn't be opened" and "image not recognized". Can anyone help me figure out what I'm doing wrong?