In the end I found that by replacing the link to the latest release with the numbers of the previous (which I found in an email on this forum) I could download the previous version from:
http://software.ultimaker.com/releases/firmware/9511/stable/5.2.11.20190503/um-update-5.2.11.20190503.swu
The downgrade worked perfectly, and now I have ssh access again in the developer mode, and I can use 3dprinteros again to print from my iPad! 😀