you can add a spatial distribution type in +netwrok_generation/add_femtocells in the switch at the beginning of the function. There you add your function to generate a fixed diamond pattern for femtocells (check the other placement functions to see how it works and what to pay attention to).
In the config file hex_grid_tilted_with_femtocells you then have to adapt LTE_config.femtocells_config.spatial_distribution accordingly.
Thanks for your guidance.
With testing several times, I found that with setting LTE_config.map_resolution to values greater than 20 for example 36, The placement(coordinate) of diamond femtocells in GUI become fix. But i dont know the reason.