A little help!
I bought an Echo Plus (1st gen) in October 2018 that came with a Phillips Hue bulb. All worked fine.
I upgraded to Echo Plus 2nd Gen with a Phillips Hue bulb and all works fine with them, BUT my previous bulb has disappeared from the app and new Echo Plus can not detect it.
Welcome to the Forums!
Have you tried restarting your Echo device and your older Phillips Hue bulb?
Also, if you have a dual-band router/modem you'll want to ensure that all of your devices are on the same Wi-Fi network band. Some smart home device only work with 2.4 GHz Wi-Fi band connections.
You can check or change the Wi-Fi settings in the Alexa app:
Please let me know if any of the above resolves your issue.