Orbi RBK12 mobile phone issue

Hi.  I set up the Orbi Wifi extender a week ago.  I have great wifi coverage throughout the house on our laptops now where as previously had none in the far side of the house.  The problem I have is our mobile phones keep disconnecting from wifi now.  My phone is a samsung galaxy 6, sometimes it is okay and other times I have to reconnect it.  When this happens I have to re-sign into the network and get the message '404 not found' , I tap use this network as is and it connects.  Sometimes it stays on and other times it disconnects again, maybe 3 or 4 times a day.  The other phone is a Samsung glalaxy 3 (ancient I know).  That phone hardly ever connects and when it does it stays connected for a few minutes and then disconnects again.  I would appreciate any help, I am not technically minded so please keep it basic.

Another issue I have is I often now get '404 not found' (occassionally I may get 401 or something else) when I look for a website that previously I had no problem with.  Not dodgy sites either that I look at.

Lastly, I am having a problem setting up Chromecast Ultra, partly I think because of the phone disconnection problem but also I think I may need to change from 5GHz to 2.4 to do it and then revert back.  Can someone tell me how to do this please.

Otherwise I will be sending the Orbi back, takes me hours to sort various issues so I just need basic easy to follow instructions.

Really appreciate any help though,

Cheers,

Faye