Getting better upload speed on FE port than GE! WTF!

Pathanamthitta, Kerala
BSNL - AS9829
DWAN - AS133301
I am on a 50mbps connection, out of which I only get 25mbps(max) upload speed that too on my FE port. When I connect to GE port, speed is reduced to around 5-10 mbps. Current setup is Netlink ONT (bridge mode) connected to Tenda AC10 router using Cat7 cable and to pc using cat6. Could this issue be caused by faulty ONT?
Yup most probably it will be the case of the faulty ont, otherwise there is no need for a ge port to act like that. By the way have u tried connecting directly to ont and check the speeds ...
Yes, I did. It is the same. It is so weird that when I connect a cat5(100mbps max) cable to GE port, it gives me better upload speed. And as I mentioned, this is only happening with upload speed. Download speed remains the same on all ports. Wonder why this is happening?🤔