I also set up IPv6 properly using HE etc as the one guy above mentioned. Maybe it can help. Who knows. But I will report back in a couple days if the problem has been fixed.
Replying because I have the exact same issue as you. Its only been happening the past few months with my ASUS RT-AX92U
I have also tried what you tried. Nothing works.
I am with Teksavvy who are using Rogers lines. I too have the Cable 120 Unlimited.
I am using CAT-8 (thats correct) ethernet...