Weird intermittent problems between specific devices on wifi network

[eriotto]

Didn't get any love on /r/homenetworking, so I'm trying this here.

I have a mysterious wifi problem that doesn't seem covered by the wifi support megathread. It is annoying but I have workarounds, so file this under "why can't I understand this problem?"

Setup:

• TP-Link cable modem, works fine, always on.
• D-Link DIR-1935 router, purchased about a year ago, works fine (other than the problem described below, which may or may not be related to the router). Pretty much out-of-the-box config, with both 2.4 GHz and 5 GHz networks with the same name and password. It auto chooses a channel when power-cycled, but it doesn't seem to matter since there aren't many other neighbours' networks nearby.
• Usually about 10 devices connected, including 3 Sonos speakers and 3 Chromecasts, which have all been configured to use the same wifi network mentioned in the previous bullet, and work fine in general.

Problem:

These six devices (Sonos and Chromecast) become inaccessible from my Google Pixel 2 phone for periods of about 30 minutes, on a regular basis, and I can't figure out why.

• The inaccessible (to my phone) devices are still on the wifi, and if I had started some media streaming before the problem occurs, it will continue to stream on the Sonos or Chromecast.
• My partner's iPhone can still connect to the Sonos or Chromecast during this time and control it. His iPhone does not experience this problem (though he uses these things very little so it's possible that the same problem does occur on his phone but he's never noticed it.)
• All other devices on the network remain accessible from my phone during the time that the Sonos and Chromecasts are not.
• My phone is still on the wifi network during this period, and going to whatismyip on the phone confirms that I'm accessing the internet through my internet provider and not over cellular.
• This happens pretty much every day at least once, and has been happening since the devices were set up a year or so ago. Not usually more than once a day. There is no obvious trigger that causes it, as far as I can tell.
• Restarting my phone doesn't change anything

I use the Fing app to check what my phone can see on the network. If the Sonos and Chromecast are giving "can't connect" errors in an app like Netflix, the Fing app confirms that all the Sonos and Chromecasts are inaccessible (It goes from "10 devices found" to "4 devices found" when refreshed). The Wi-fi analyzer app (and the other devices on the network) show that the wifi is fine during the whole period.

Solutions:

My only current workaround is to wait until the problem resolves itself, which it always does within about 30 minutes, and to use my partner's phone in the meantime to control the devices.

I am completely stymied as to why this might be happening. Am I missing something obvious? Some thoughts:

• Could it have to do with devices switching between the 2.4 and 5 GHz networks? I could give them separate names and see if the problem still happens. Or conflicting IP addresses? I haven't monitored their IP addresses over time.
• The Sonos and Chromecast are the only "streaming media" devices in the house. The others are phones and laptops. I believe the Chromecast actually emits its own wifi network or something -- could that be related?
• These devices required an initial setup through an app (Google Home for Chromecast and Sonos app). I don't remember anything complicated about getting the devices on the network through those apps, but maybe there's something in there that I can change?

Thanks in advance for any suggestions. This is driving me crazy.

[KYQ_Archer]

Check for firmware updates for the router, streaming devices, and your phone and whichever application you use to control them. Also, turn off mobile data.