Inspiron Direct Wired to Router Slow Download Speed - Direct Wired to Modem OK

Spectrum download speed 118 mbps is normal.

I have 2 computers w ethernet cables direct to router, W10 and W7 - W7 is fine

Inspiron Core i5 7th gen W10 direct wired to router is only 9 mbps. Same on Wifi. 

Firewall on or off makes no difference

All network adapter drivers up to date

Troubleshoot network adapters all normal

Reboot router no difference

Swap cables with other computer, no difference

Spectrum says its a computer problem

Wifi 2.4G to this computer is only 9mbps

Wifi to other devices is normal 

I have changed routers 4 times, each time download speed is normal, then about a month later this problem starts again. If I replace the router, it will probably fix the problem again short term. 

IT person says replace the network adapter

Look forward to advice,

Hal