I'm having the exact same problem.
I have 2 E27 RGB bulbs that have been working fine for weeks. No firmware updates available.
Recently, they keep going offline. Rebooting them sometimes fixes them and sometimes it doesn't. Sometimes rebooting the router fixes them.
DHCP lease on router is 1 week. Bulbs go offline after just 1 hour of rebooting them/the router sometimes, so I'm sure it's not related to the DHCP lease. Sometimes one is fine and other is offline. Sometimes both at the same time.
All other items on my home network work fine when lights are offline.
When they're offline, I cannot ping them. Schedules also stop working.
Using Singapore server and the yeelight app.
Router is a Netgear D7000