Wifi/Wired connections to TM's router is getting 300Mbps, but my second router is only receiving 100Mbps

My router's firmware is showing its receiving only 100 Mbps, tested on speedtest also same result

TM's router's firmware showing its sending 100 Mbps via LAN port

What do these findings indicate?

If I'm getting 300 Mbps directly to TM's router, it should also serves 300 Mbps to downstream routers connected via LAN port? It should work like this isnt it?
All cables are cat6 up, except I dont know whats inside my cat wall jack behind the walls
Went to TM's routers firmware and I couldnt find anything related to "speed"

If TM's router is indicating its sending 100 Mbps via LAN port, its safe to assume theres no faulty cable inside my walls right?     

But TM's router is already indicating its sending just 100 Mbps, isnt it suffice to eliminate the possiblity of faulty cable inside the walls?

Hmm what cables do i need to test with?
Because the problem has narrowed down to either the walljack on first or second floor, or the cable inside the wall that connect both walljack

So its not really about cables at this point isnt it?