I moved to a new house & wanted to set up multiple IoT devices for Apple HomeKit. I use Meross switches.  Have several HomePod Minis & a couple AppleTV's.  When I started building the network I set the main network as 5GHz only (for now) for now & gave it a unique name. I turned on IoT Network & gave it a separate name, 2.4 only.  Initially, I could jump on either the primary or the dedicated IoT Wi-Fi's without an issue.
 

I kept my phone logged into the IoT (2.4) & "forgot" the primary 5GHz to make adding devices to Apple Home app easier. 

The AppleTV's are hardwired in. 
 

I added all the HomePods on to the Home app using the IoT network on 2.4GHz only. Then I began adding all the Meross switches & Hunter ceiling fans (these are a PITA, IFYKYK).
 

I then began putting the Meross switches in to WiFi pairing mode & adding them one by one.  I would add one, then insure under Connection Preference that "2.4GHz" was checked.  After adding ~ 11 of them I could not get any of the others to add. Despite following similar steps as the first 11, the others do not connect to the Home app. 

1-2 of them have stayed in WiFi pairing mode & I can see the network the switch broadcasts under my WiFi settings. 
 

Here is where things get very odd.  My WiFi began to drop intermittently. Every few mins I was unable to connect to either the primary (5GHz) or the IoT (2.4GHz) wifi's. And now, when I do, I get the message "No internet connection."  In the Deco app there is only minimal network traffic to confirm nothing is connecting. 
 

I am at my wits-end trying to figure out why the WiFi keeps cutting out. I have tried rebooting the Deco's, hard resetting my phone, etc. I just don't know how to get the WiFi to stabilize. Wife came home & she cannot connect to wifi (primary or IoT) on her phone, laptop, anything....All wired devices are doing fine.