Bulbs go offline after some hours


I used for 2-3 years now these two bulbs (YLDP02YL) without problems but since a few weeks they go offline after a few hours after switch the power off and on. I have also they yeelight led strips and those will stay online all the time.
I changed already another router, log-out and log-in to the yeelight / Mi Home apps, delete and re-added the bulbs to the app but nothing helps.
Can you advise what to do?
Mi-ID is 1776852363 and bulbs are on FW 1.4.2_0076