Archer C6 Gigabit Router

Hi All, need some help please

I have a 250 MBPS connectivity from ISP, when I connect the ISP’s internet cable to My desktops LAN port - which is the default LAN port from my motherboard - I get full 250 MBPS connectivity.

I recently bought a TP Archer C6 router with Gigabit connectivity and Gigabit ports. Through the 5 Ghz frequency I am receiving full 250 Mbps speed on my iPhone, however when I connect the desktop to the router via a LAN cable, my speeds are restricted to 100MBPS. Any reason as to why this is being capped at 100 Mbps? Any change in setting I need to make please?

It’s strange when I connect the ISPs internet cable directly to my desktop I get full 100% speed but not through the router - C6 Archer claims to provide full gigabit ports. Unsure what I am missing

Kindly help me out - Thanks in advance

SC