Here is the only way I could get all of my devices to connect again after i tried making some changes and screwed it up..At one time I had 4 Hue bulbs and my thermostat connected.
I went to the Smart Home and clicked each device, 1 at a time. After I clicked the device, I hit Edit in top right, then pressed the trash can to delete the device. After all my devices were deleted, I did a hard reset by pressing a paper clip in the reset hole on the bottom of the Echo plus for 8 seconds. At 8 seconds, a chime is audible to verify reset. Then I did the initial setup again to connect to my wifi. Then from the Smart Home screen I tapped the question mark with a circle around it in the top right corner and scrolled down to RESET PHILLIPS HUE LIGHT. I clicked that and entered the light bulb serial number and pressed continue. It reset my light and was able to discover it. I had to do the same process of resetting and discovering for each bulb.
Now all my bulbs and thermostat work!!
Good luck