@Dan_C. 

My router is a Fritzbox FttH router.

To test your theory, I've installed DHCP on my home server and disabled it on the router.

This gave the same effect (few hours good)

On the DHCP server, I don't see any errors in the log.

@TP-Link Deco 

I'm have deco x20, switch LS105G and IPTV hypptv from unifi.

Can use this matter ? 

@TP-Link Deco 

Hello,

Maybe the problem for my specific switch is already answered, since this thread is very long a didnt read it whole.

So im having issues connecting my Deco M4's via ethernet backhaul through a switch.

Ive got a Cisco SG200-26 and could get it to work (btw: direct connection work fine, so the M4's are ok).

Ive tried connecting the to the switch without any configuration, as well as connecting them on a seperate VLAN - same result: the M4's connect via WiFi.

Any help/tipps?

BR

mihi

@TP-Link Deco 
I got a question about setting up the deco's all wired after a switch.

Is it possible to connect the deco's this way:
From the main deco UTP cable to Switch then UTP cable to Slave 1 and then UTP (out of slave 1) to slave 2?

@Deackon 

Yes, this connection between Decos will work just fine.

@TP-Link Deco 

Please help ,

I purchased my setup only 2 months ago , earlier when i set it up everything worked perfectly and like a charm 

I have connected all of my Deco device through Ethernet Backhaul , 2 are Deco X60s and 2 more are Deco M5s 

Recently i faced an Solid red light issue on one of my deco devices , and had to call TP-Link India Support , it was a hellish experience , as i had to wait on hold for 15-20 minutes only to be connected to an "Engineer" who had no knowledge of the deco systems , and would be assured that a call back would be arranged in an hour or two to sort my issue . It has been a month since my first call i am yet to receive any such call-back .

After several days of calling and facing the same issue , one of the support staff tried to help me out , and through troubleshooting hit and trial , we came to the prognosis that i have to switch my deco network to an access point mode for this issue to be resolved . And it did get resolved , but in the days leading up to today i have been facing slow speeds on my 2.4 Ghz network and frequent and random falls in connection in devices , it it more noticeable because , it usually can take a whole 2-3 minutes to get internet connection back on those devices . What usually happens is , i will be watching content or doing research , the device will slow down , then when i go into the the network settings of the device it will show connected , i then run a speed test and it will show an error , then the device will loose connection . Then i open settings to see what has happened , it will show disconnected , i click on the network name in my settings to connect again , it will either say " failed to connect to the network " or connect and give very slow speeds of 0.01 mbps on the 2.4 band all while signal strength is strong to excellent . 

I don't know what to do i have done basic trouble shooting , doing network optimisation from the app , anything and everything i can do i have done , but this does not happen once or twice a day it happens multiple times and each time i get flustered , and regret spending soo much money on a deco system . All day as soon as the service centers open , i try to call the TP-Link India Support , they just give me the same story of a miraculous call-back and i am left in despair , felling mentally drained , and regretting buying TP- Link Deco system . 

One more thing i noticed is that the speed issue is overall prevalent on the 2.4 band  i turned off the 5ghz band to test this theory , and turns out i was right , when 5 ghz is turned off , most devices only get speeds of 20 mbps max not above that , earlier in the router mode there was QOS to handle this but now in access point mode what do i do ?

This is a video of what happens when the issue happens

@TP-Link 

ANSWERS

Approx 40 devices max on the network in day day time when everyone has classes and work 

Bandwidth subscribed 150mbps up and down 

Will try suggestions and update you in a day or so , in the meantime , the 4th deco (office) that i had setup again on advice from phone help engineers , and switched my entire network from router to access point mode has stopped working , 

It was working till yesterday , but not it is red solid light , tried rebooting my entire network and modem , nothing seems to help . How to proceed ? 

I have not changed anything in my network as of yet .