The issue was caused by two cables failures. I tried testing for issues with the cables, but because two cables were causing issues (and not just one), I did not see it at first. I finally found out...

Model : Hardware Version : Not Clear Firmware Version : ISP : My local network uses a "TP-Link ADSL2+ Gigabit Double Bande AC1750". It has three computers: two connected by ethernet (let's call them...