Ethernet LAN disconnects after turning off kitchen tubelight or fan
Ethernet LAN wire coming from local internet provider


Your profile says you're in India. It also sounds like your ISP has an Ethernet wire that runs into your dwelling (I suspect this is an apartment). I further suspect they ran the cheaper Cat 5e inside the building.

Fluorescent lighting produces a fair amount of electromagnetic interference. My bet is that the Ethernet parallels either the light fixture or the electrical wiring in general (I could easily see the latter being done to save costs by cutting corners).

Unfortunately, since you didn't run the wiring yourself, there's no easy fixes here. If you had run them, I would make sure you keep the Ethernet away from the electrical. I would also run Cat 6, since it affords better shielding. Putting the wire inside conduit would also offer better shielding.