@Florida-Guy 

Be sure the antennas are secure.

Use separate ssids for each band.

After changes to the router settings it is a good practice to reboot it. Some settings do not take.

Check the security wifi setting as it may be to high for your devices.

May need to adjust the wifi band width.

You may need to delete (forget) the network settings on the device and reset it up especially if same as previous ssid.

Power cycle the devices.

If the device allows for more than one ssid such as phones and tablets, set it up to use just one network band. 
Some iot only work on the 2.4 band such as many cameras. 
Some devices require specific addresses so be sure to add to the reserve address table and reboot the router. 
If using a reserve address table, some router require the address range be in or be out of the range of the other devices to work. 
Some devices may require ports to be open using options like port forwarding for items like security nvr.

Side note: have seen strange things happen when the power plug that came with the router is not used. The previous router power plug may fit but not have amps for the new router.