@Labib_T_Alam If your device is provided to you by your ISP then contact them to be advised further. This model FW has been customized for ISPs.

@Jondas Would you mind tell us what's the make and model of these problematic Android devices ? Meanwhile try first post suggestion from this thread and see if there's any improvement.

@Kragdob What most probably happens is that with VPN client on your Android device you're bypassing the Parenatl Control feature. Exclamaition mark means that you can't resolve the websites IP...

@Jondas Try these wireless security settings: If this doesn't improve things, set these DNSs in Internet connection settings: If the issue persists then use this article to select less crowded...

@PRATUL If your router has been provided to you by your ISP and you don't see a Gest Network under Advanced tab, most probably its FW has been customized for your ISP.

@mfran2002 Glad that you've sorted this out...) Cheers.