whats more baffling is why is your internet at 1.2Mbps in the first place? your current speed is normal if you got a 50Mps plan. when you replugged in your ethernet cable, your internet connection settings refreshed automatically to its correct settings. someone mustve resetted your router as well, but you didnt refresh your side of the internet or been awhile since you had your last restart. like what @kanewolf mentioned. your cable connector might have some lose wires on it. you may want to purchase a new ethernet cat5/5e/6/6a cable.