I've made the first draft of a new website. I can do a device preview on an iPhone and an iPad when using Chrome or Firefox. But when using Safari I got a blank page. I copied the whole stuff to a test folder on the server and then Safari had no problems at all. Every preview on desktop browsers work (also Safari). So I suspect it a problem with Safari on iOS or a specific device preview problem.
Which version of Safari?
On my Mac, I'm running v13 (don't know exactly because it's off now but I have the latest macOS installed), on iPhone X it's iOS v.13.6 so it should be Safari 13, on iPhone 6 it's iOS 12.4.8 so Safari should be v12.