Just to add another wrinkle to this: my Yeelight bulb will respond to the app and to LAN TCP calls made from a Python program according to the LAN specification. But intermittently it fails to respond to SSDP discovery requests as I reported in Intermittent responses to SSDP requests. Even at the times it fails to respond to SSDP requests it still responds to the app and to direct TCP requests. I’m using a Linksys router, and I have no problems with any other devices at any times.
I have been trying to build a Mycroft (like Alexa, only open source) digital assistant skill for the Yeelight bulbs. The failure of SSDP discovery to work means that I can only post it with a rider “intermittently breaks due to problems with discovery unresolved by Yeelight” and that isn’t very satisfactory!