Why are you using a ADSL equiped modem router with a fiber optic cable connection.
Have you swapped from an ADSL connection to a fiber optic one recently.
And want to use the old ADSL as just a router ?
If you bought the router recently, and still have a receipt for it take it back to the shop or where you bought it from and by one for a fiber optic cable connection. Cable modem router version from TP- Link. C7 is for fiber optic cable connection.
You want this model number of TP-link router for fiber optic connection.
TP-Link Archer C7 AC1750 Wireless Dual Band Gigabit Router
It would make things more easy for you.