Unify excel date format for all users

At work we have a file, that used to be set up in a way, where no matter what date format you use (dd.mm.yy ; mm.dd.yy ; d.m.yy ; m.d.yy ; dd/mm/yyyy ; dd/mm/yy ; ...) it would always visually adjust to the current users system settings and always read the input as date.

So if one coworker used and typed dd/mm/yyyy, he would see all dates in the table like that.
While I use dd.mm.yy and I would see all date like that.

I hope the explanation makes sense...

this stopped working last year, and I tried what internet suggested:
Changing the user settings in OS to a format they use.
Changing it to a adjustable format so everyone has their own format
and now it is all set to a firm format (dd.mm.yy) so it displays the same for everyone and it doesn’t matter if they type it with slashes (dd/mm/yy ; dd/mm/yyyy) but if someone types specifically in dd.mm.yyyy format it doesnt recognize it as a date anymore...

I feel insane at this point!