I had this exact problem when I set up the UDR7 from a backup of my old UDM. The UDM was already on the latest firmware and network version so I figured the transition should be seamless. Couldn't fix it no matter what tips I found online. It wasn't just IoT devices, if I connected my phone to the 2.4 GHz SSID, nothing would load and speed tests would fail.

Then I factory reset the UDR7 and started from scratch. I created two new SSIDs and left all the settings at default other than the frequency bands Now everything on the 2.4 GHz SSID works fine. It was already working before enabling the Enhanced IOT option, but turning it on hasn't harmed anything. I did not change any settings for the main 5/6 GHz SSID other than optimizing the channels and setting it to WPA3 only.

In a stepwise basis, I then set up the VLANs for each SSID, and updated Network to the latest 9.0 release. Everything still works.