Either you have dysfunctional wires on the house either is problem with your internet provider. Try contact them.
I had a similar issue. Random disconnections and sometimes tend to happened very often. Tried many things like you did and nothing worked.I figured out that the cables on the outlet where messed up due to a water leak from the upstairs neighbor.
I changed it to a new outlet of the house which was in different cables and the connection got a little more stable but not much better.
After contacting my internet provider, they sent a technician and he told me that it most likely problem from the provider. Currently im in the processes of changing internet provider.