Internet not working on mobile devices mainly apps

Hello All,

I am facing a weird issue with mobile devices (mainly android) connecting to internet.
below is my network diagram

View attachment 18663

Issue 1:
* when my mobile device connect to main router AC86U it says connected but none of the apps works for ex: facebook, youtube, whatsapp, news, speedtest
* i will b able to open chrome and browse without issues
* this happens mainly on Android devices (mobiles and tabs) , ios, laptops works fine
* if i put private dns on android...

Internet not working on mobile devices mainly apps