I have recently had problems with smaller IOT/appliance-type devices connecting to my DECO W7200 router. This all seems to be 2.4GHz connections. They mostly complain that the password is wrong, but I think this is just a general message about can't connect.  

For the record, the hardware version on the devices is listed as US/1.6 (not available in the popup above).

The Primary WiFi is configured to 2.4/5GHz,  and the security WAP2/WAP3 is personal.

The Guest WiFi is configured to 2.4GHZ only, and Security  WPA2/WPA3 Personal (but I have tried WPA/WPA2, WPA2, and None) all seem to fail connections. 

The devices: 
 

1) Ring Gen 2 door chimes.  I have two.  These used to connect fine, but now both refuse to connect.   I tried using both the Main Wifi (2.4/5GHZ) and Guest WiFi (2.4GHZ only), and guest Wi-Fi was iterated over the different security encryption settings.   These devices only support 2.4GHz connections.   I have tried reboots, etc, in between settings changes, too.   Replaced with Ring Pro devices that support 5GHz, worked on the first try. 

2) SimpliSafe base station (Gen 2) also 2.4GHz only.   This device will eventually connect to both Main and Guest wifi, but you have to try like 2 or 3 times before it will connect.   This used to just connect immediately.

3) Brand New Brother Printer HL-L2350DW (supports: IEEE 802.11b/g/n, which appears to be 2.4GHz only to me)  would not connect to either the Guest or Primary WiFi.   The error message complains of the wrong password again (it was correct).   However, I also have an Xfinity-branded XB7 router in the house, and the printer was connected to that on the first try.  The Xfinity device is configured with dual 2.4/5GHz bands.    Connections to the deco device were tried with the Security Setting at WPA2/WPA3 Personal.   I have had enough failures already, so I did not vary the security for this one. 

Most of the devices in the house work just fine—iPhones, Apple TVs, Raspberry Pi, etc. But these smaller appliances are having problems, and if it were just one, I would say it was a bad device. But I have a whole host of them that can not connect. And the SimpliSafe and Ring Chimes used to work with no issues.

I ran the network optimization utility in the Deco app. It found nothing in and around where these devices are connecting. All areas have a strong WiFi signal, and most devices are within 10' to 15' of the W7200.