Certificate problem

Hi,

Cisco Umbrella certificate will randomly block access my internet on Iphone devices. Even i don’t know what is it.

The problem started about 2 months ago. I've noticed that some sites would randomly not open on Iphone also my application’s doesn’t work IG,FACEBOOK, Mail etc.

What's strange is that after turning WiFi on my phone, and not using LTE the problem disappears.

I have tried everything .. :/ couldn’t find any solution.

please help