@Pavin 

Sounds like you cable between the router and modem is the problem. Either 'bad', poor contacts, or not CAT5e or better.

@IrvSp I'm using brand new cat6 cables.  the same which worked fine during the modem to pc test.  But, what if the incoming port on the router is flawed?  how would i check/fix that?

@Pavin 

If you've gone through every cable available which is commonly the reason why, check the firmware on the router to make sure that is up to date. Compare the build number (which is actually the date) you currently have with what is on our official website, If your router is up to date, I do a factory default on the router by holding the reset for 15 seconds, then test the speeds again. Lastly, I would reach out to support.

@IrvSp So I followed your instructions exactly and I did get a different result.

the modem ethernet light was flashing green instead of orange, which means a Gige connection with the router.  Cool.

BUT

router internet light was orange.  it didn't think it was connected to isp.

I reset the router, no change in outcome. (btw firmware build 20180125)

Then i swapped to my original patch cable between the two and started over.  Got a Gige connection, and the router got an internet connection....checked the speed and now Im at 25mbs, hardline and wifi.  

I'm a day away from giving up on TP-Link.

@Pavin 

Did you change ISP's and/or Internet Speed???

Possible your BANDWIDTH CONTROL is ON or you have the INGRESS/EGRESS speeds entered and they are too low.

@IrvSp  tested speeds are super low now. Under 25mbps. The router is reset to factory. I rechecked my speed without router and it's still 945mbps. I'm going crazy. 

@Pavin 

I've not seen an answer to 3 of my questions?

- Did you power cycle the modem and router properly after changing modem connections to router or PC?

- Did you change ISP speeds or ISP's?

- Did you check Bandwidth Control?

@IrvSp 

yes.  The modem ethernet port flashes green now, indicating a 1GigE connection to the router.

No, same ISP but faster speed, from 300mbps to 1000mbps.  Hence the new modem.

Bandwidth control is off because NAT boost is on.  However, i did turn NAT boost off and enabled bandwith conrol, setting it at the max allowable (1000000).  didn't matter.

today, I am getting speeds ranging all over the place but all under 100mbps.  I also get different test results at speedtest.net vs xfinity vs speedtest app.

it is all garbage.  the upload speed is the only constant reading (42mbps) thru all this bs.  I've checked other posts with virtually the same issues, no answers...except to get a different brand.