My Deco M5 satellite cannot get past 90mbps using ethernet backhaul. My full speed is 500mbps and the app speed test shows even more, 750mbps generally. 

So I did a speed test recently and the speed was at 90mbps. I began looking on the connectors, switched both and somehow it just stopped working. Found a cable tester, confirmed that it wasn't working (a couple, different colors) and went for a new cable. My belief was that the cable was at first slowing down the speed and then it just failed. Bought a new CAT6e cable and connectors, got it all the way from one Deco (main) to another (satellite), connected them both and... 90mpbs again! I do believe that the old cable just stopped working probably due to those new connectors with some error and was probably still good, but that's (25 meters of) water under the bridge right now.

Since that got me kind of startled, I decided to do a speed test directly from the main Deco. I removed the cable from the satellite, connected to the ethernet port of my PC and... 800mbps!! So, the cable is ok, internet speed is ok, but ethernet backhaul is not.

I've looked for similiar problems and the only one I found here was a bad cable connection, but the cable is a brand new CAT6e and it is sending full speed directly to the PC. I can't get my mind around that I would need another cable to make the Deco M5 satellite to "accept" the actual full speed.

Any thoughts?

Thanks in advance!