Similar issues with suggested workarounds, sometimes solved by ordering a new device:
- What's wrong with the WAN port (MT7981) - For Developers - OpenWrt Forum
- GL-MT3000 WAN interface eth0 going up and down - #6 by T480s
- WAN connection continuously drop (GL-MT3000 Beryl AX)
- MT-3000 intermittent disconnection
- GL-MT3000 - problems with stability after upgrade to v4.5.16?
I notice that a number of them mention using a "Freebox" as upstream ISP router, which is also my case. So maybe there's something Free.fr does with their firmware that interacts badly with MT 7981 driver.