1 month ago my house received a new connection to fiber broadband (1 GBPS) and I purchased a Deco mesh system (Deco X60 from Amazon, that model doesn't show in the model selection dialog on this post). I have 1 Deco in the basement connected to the incoming fiber, and the other 2 upstairs. None have wired backhaul because I don't have the wires, but I have full signal strength throughout the house. (single story house with a basement, about 1800 SF per floor). Everything was great for the first few weeks.

Prior to the fiber connection, I was on Xfinity, using a 6-8 year old ASUS router which reached all areas of the house with good signal. 

Starting this past weekend, my wife and I noticed issues that so far seem to only be with our iPhones: web sites will fail to load completely for a very long time: 1 or 2 or 5 minutes, or until we give up and do something else. For example, I will open the YouTube app and it will show a blank list with no thumbnails, or the Netflix app will get stuck on the home screen without showing any video thumbails, or a browser page will try to load but will be stuck with Chrome's little blue progress bar at 10% for minutes at a time. Sometimes the page will load eventually, or sometimes it will load if I force quit the app and try again. If it's a browser, there will be certain sites that won't load, but others will.

We have not noticed this issue on PCs, only mobile devices. Other hardware connected to the WiFi, such as smart TVs, haven't seemed to have any problem. 

When this happens, if I immediately switch to the Speedtest app and run a test, I am getting 250+ MBPS every time, and other apps and pages will load, it seems to be just random ones that don't. Also, if I turn off WiFi and use my cellular data instead, everything loads immediately, so I'm pretty sure it's not a server side issue.

When this first started happening 2 days ago, I ran a firmware update and reset the Decos, so I have the latest firmware, but the problem persisted.

What else can I check to troubleshoot this? I'm thinking about returning the Decos to Amazon and getting something else. I don't know for sure if it is even the Deco though, it could be something with the phone, or could be the broadband I guess, but it seems like with broadband you either have internet or you don't.