5g home internet - some sites won't load

Try taking your gateway's wifi from "auto" to 2.4 GHz only.  This resolved the same issue for me.  For reasons I won't bore you with I was all set up from switching from the old original gateway to the new white 5G version so I could get an external antenna mounted on the roof.  I'm somewhat rural and setting my gateway outside was a game changer for BW to my address.  So upgrading to the new gateway was sort of a no brainer.  Anyway, having gone thru the issue of setting up some thermostats that don't use 5GHz wifi a week ago when I received the new gateway, when I started having issues loading websites today, such as costco.com for example, I remembered I had reset my gateway earlier today but forgot to set to 2.4 GHz only.  I did that, and voila, costco.com and other sites started working again.  Hope this helps someone else.  On the app, go to the "network" tab, and click on the SSID, and in the "update network" sub-menu, under "advanced settings", you have a pull down and can select between Auto, 2.4 GHz and 5 GHz.  Select 2.4 GHz.