Make sure “Include Standard” Files is selected for Cumulus MX. From build 3042, MX automatically uploads the realtimegauges.txt file. Note that the realtimegaugesT.txt file for MX is slightly different to the one for Cumulus 1, and it is important to use the one supplied with MX for MX.
You have now told Cumulus how to update your web space with the [[Customised_templates#The_Standard_Templates |Standard Cumulus Template]] files for the web.