If I’m understanding your situation properly, I recommend you explore using the Limited Schedules option until the properties have reliable WiFi.
Essentially, this would allow you to temporarily set up the controller as a “dumb” timer using one of 12 pre-sets.
While you could do as you suggest with a hotspot, the controller would only be connected to the Rachio cloud for as long as your iPhone 6 is giving it a WiFi connection. It would go offline as soon as you, and your hotspot, go out of range or otherwise disable the WiFi.
To clarify: Limited Schedules won’t allow for remote access, but will at least ensure the landscape is getting regular waterings. Once a regular WiFi signal is available at the site, you or your clients can take it out of this mode and set up as normal.