@coolspot18 do your 2.4/5GHz and 6GHz networks have the same SSID? This has been known to cause similar problems, particularly on  older iOS devices. Try giving your 6GHz network a different SSID to see if that fixes it.

If your new network and your old network have exactly the same SSID it might be worth "forgetting" the old network on your devices and signing into them fresh to renew the saved credentials on your devices.

  @coolspot18 can you link to where Apple recommends naming two discrete networks with identical SSIDs and passwords? I would still recommend having two separate SSIDs and having your 6GHz capable devices on this network and keeping your other devices on the 2.4/5GHz devices. That's clearly the way Deco have designed their product and this solution fixed someone else's iOS connectivity issues just yesterday.

Are your IoT devices connecting to the main network(s) or the specific IoT network? If it's the main network then I still think that is the most likely cause.

A reboot might be "fixing" the issue because your devices connect to the first network they encounter, then as you roam about the house or the devices try to locate the best network they hop to the other network on a different frequency (or maybe even different encryption method i.e. WPA2, WPA2/3 or WPA3) and causes it to fail. It's worth troubleshooting to see if that is the issue.

  @coolspot18 I personally disagree with Apple's suggestions, as they don't know how every router is designed or how every mesh technology is implemented. Some Mesh networks might work flawlessly across all 3 frequencies with identical SSIDs and others might not. It's also not a security warning, it's a "compatibility" warning. That implies that it's not safe or less safe to have separate SSIDs which is not the case.

You also didn't say if your devices are connected to the main network or if you have a specific IoT network set up for them. If they have their own IoT network and separate SSID and they are still being disconnected then the Deco units could be faulty or maybe the WPA version is set too high for the wifi standard the Wemo plugs are using.

I don't really have any other suggestions for you other than testing this theory out unfortunately.

  @coolspot18 the two main benefits are:

1.  You can set separate encryption standards or exclude the 5GHz frequency from the IoT network. This enhances compatibility for IoT devices (typically limited to 2.4GHz for power efficiency and often don't support newer wifi connection protocols hence the option to disable Mesh technology in the Dceo app as well) and older devices like my Canon printer which won't connect to my WPA3 main network. The IoT network allows me to connect my printer via WPA2 and the fact that they're on the same VLAN means I can still print wirelessly without the cloud. Arguably redundant with WPA2/3 available but I prefer discrete SSIDs for network management.
2. Device management, it's sometimes easier to have separate SSIDs to keep track of devices, especially if you're setting up a network for the first time and everything connects with a generic name.

And all those years you were using a single network you were also having issues with the Wemo sockets as well? I appreciate that you want a single network but it might not be possible/stable on the Deco system which is why I still suggest you try a new SSID to eliminate this as the issue.