@sjeeves2020  Not sure if this helps or if u have fixed. I have a deco4 and couldn't connected to firetv box (sticks worked fine), I reset box and connected with cable, after removing cable wifi dropped off again. I then selected join using WSP went into the deco app and selected WSP, chose the closest deco, it failed first time but worked second attempt  all good now on WiFi. Hope this helps.

@Pjb1 hi, thank you, i cant hard wire them as not next to D9, in different rooms... I put guest network on, only 2.4 ghz and that seem to work... so i can keep seamless on main network on. Not sure if update sort this out... or a amazon update. have to see...