Problem with https authentication hotspot help

I have a problem with the login page at the mikrotik hotspots.
The problem is when users try to log in for the first time to get the splash page skipped.
If you try to access via an https page, for example https://www.google.com, the splash page does not jump. So to solve it, we have uploaded a certificate of COMODO in the mikrotik and in this way what happens is that a warning goes by saying that the page can not be secure. Especially on iphone devices.

Is there a way to avoid this warning that the page may not be safe? Not all users will press continue if this appears. For me it is a great problem.