@GS-Link 

Is the TP-Link model number Archer AX90? Please verify this by referring to the device bottom label.

If you want to add the TP-Link router to the BT Fibre via ONT network, firstly, ensure there is already internet access from this BT Fibre via ONT, you can connect a computer directly to the BT Fibre via ONT, ensure you have internet access on the computer. If there is no internet, please contact BT support for help.

If the internet is working fine, then connect the TP-Link router to the BT Fibre via ONT, then login to its web interface page to choose the correct WAN connection type provided by BT, then follow this guide to set up the internet:

https://www.tp-link.com/support/faq/1556/

@GS-Link 

Great to know you figured it out.

I think it is similer to this article: https://www.tp-link.com/support/faq/2942//