No correct mobile display

[GK5]

Hello,
I'm trying to create a new website that must have mobile variants.
I have two variants:
- Main 1024px browser 752px and above
- Mobile1 480px browser up to 752px

When using Preview in Xara, everything is OK.
But when loading the published version in a browser I have some strange effects on one of my mobiles:
- On my computer: 27 inch monitor 1920x1080 >> OK: main
- On my laptop: 15 inch 1920x1080 >> OK: main
- On my old mobile: 5 inch - 1280x720 >> OK: mobile1
- On my new mobile: 6,8 inch - 2460x1080 it is not working correct.

The first screen is OK but when selecting the newt screen in the menu, the display seems to be a mix of the main-version and the mobile version (see screenshots)

I think a have to create another variant but with what number of pixels.
I've already tried 1080px but this is creating a variant that is much bigger than my main variant.

Someone knows a solutions?
Thanks
Frank