@Rtiamlee 

I would say, the problem is with the Ethernet cable.

Deco M5 has gigabit port. Looks like it can't negotiate gigabit and decided not to negotiate 100Mbit, switching to WiFi backhaul instead. You tried another M5 and got same result, so it is consistent M5 behavior.

Deco E4 has 100Mbit port. It successfully negotiated 100Mbit.

Laptop - you didn't say what is its Ethernet port speed. Many laptops have 100Mbit, so it'll be not different from E4. If yours has gigabit port, check if it did negotiate gigabit speed and run speedtest to see if indeed laptop gets faster than 100Mbps download speed.

Try reconnecting that Ethernet cable to a different switch port and make sure connection is tight. If you are lucky, it may fix an issue. If not, consider replacing cable.

>is there a possibility that the slave deco 1 was too close to main deco so it would use wifi instead of the Ethernet backhaul.

Extremely unlikely. I've checked M5 logs (for different conversation in this forum) and found it explicitly says in logs "found Ethernet backhaul, disabling all other backhauls."

@Alexandre. 
 

Thank you for the quick and informative response. 

So as an experiement, I tried switching the main deco from my Kitchen to the living room (the one with the suspected bad ethernet cable). What happens is main deco gets super slow speed, kitchen slave deco 1's signal source becomes wifi.

Do you still suspect cable to be the issue in the living room? 

Thanks

@Rtiamlee 

Yes.

Consider replacing that cable.

I agree - it sounds similar to what I had experienced on my setup.

In my case, one day my slave decos started to randomly drop (blinking red) but master would always be online. They would repeatedly go from green to blinking red 1-2 times before finally becoming stable green (until the next time they decided to drop). This could be anywhere between a few minutes to days. Occasionally the slaves would report being connected via WiFi backhaul and the speed would obviously be significantly below what Ethernet backhaul (due to the long narrow layout of the location). Sometimes, the decos would report Ethernet backhaul, but speed tests on clients connected to the slaves would always yield sub 100Mbps, while the deco speed test would always report full speed of around 675Mbps.

Key part is here - I used a network tester on all my cables repeatedly and found no issues... until I tested my shortest cable while holding both sides of the network tester in one hand while looking around at one of my switches. At certain angles, due to the stiffness of the CAT5 (it's an outdoor cable with stiff sleeving) one of the wires didn't have proper contact - the CAT5 cable was about 15cm long. After replacing that single cable, no drops, no slow speeds.