Hi,
Please check your cable, maybe instability is caused by the cable being broken. Check if your RUT955 doesn't have some kind of auto reboot or strange DHCP lease interval configured, for that, I would suggest resetting the router fully.
Have you tried swapping RUTX09 and RUT955? That RUTX09 would give Wired WAN to RUT955 and see if that somehow stabilizes the connection?
EB.