R6800 Iphone connnected ti wifi but gets no internet

I am experiencing a lot of issues with Iphones withthe R6800 router.

I am ocnnected to the WIFI, no error message, but I don't get an internet connection.

Windos laptop and Andoid phone have no issue.

It is with an Iphone 6, 8 and 11.

The R6800 is installed after the modem in a router configuration.

What do I need to do configure to get internet access?

I jhave already disabled IPV6 and QoS

Thanks 

David