@naelgh 

I have the same issue with X20 ( X20-DSL FW1.013 Build 20231226 Rel. 64412  , 2x X20 (v3) FW 1.1.8 Build 20240119 Rel. 37701) 
Probably the problem is related to network ip from 192.168.0.xx to 192.168.4.xxx
The workaround is to change the subnet to something above eg 192.168.7.xx 
But not all of us can change the subnet - like me eg.

I read many articles and found that some fixes were delivered in newer firmware but I have the latest and the issue still exists.

What is strange in my case -> everything was ok when I used only one Deco but when expanded to 3 devices it stopped to work.
Once I added some device for reservation eg 192.168.1.45 after a few seconds it automatically changed to 192.168.0.45 and wont apply to target device.

I hope the TP-Link will finally deploy the fix  for all Deco devices. 

@David-TP - I saw you helped before in the different thread - suggested some firmware for different HW versions - Can you help us as well please? 

  @JouJek 

It maybe worth adding the reservation to the list but also manually setting the IP to match.   Its not ideal but the problem is historic across many TP Link devices.

  @amtrakuk 
Thank you for yyour feedback.

I have tested many many many diffrent scenarios... including suggested by you. 
Even on the list the reservation looks fine the device is not reciving correct/reserved IP after restart.

All works fine on 192.168.68.xx, 192.168.100.xxx --- in my case only subnts from 192.168.0.1 to 192.168.3.255 are affected -> rest looks fine.
 

I already contacted the TP Link support but they cannot reporduce the issue in the Lab... 

In my case the only workaround is to change the subnet and IPs.  Because I have a lot of CCTV cameras with staitc IP alredy assigned that is a bit more deal to manage with external company ;) 

JouJek wrote

 All works fine on 192.168.68.xx, 192.168.100.xxx --- in my case only subnts from 192.168.0.1 to 192.168.3.255 are affected -> rest looks fine.
 

I already contacted the TP Link support but they cannot reporduce the issue in the Lab... 
 

  @JouJek 

That same issue has been discussed 2 years ago, here: DHCP address reservation does not work correctly.

This issue should be known to TP-Link Deco developers. It is possible the bug has been reintroduced in newer firmware update, but still the issue is not new or unknown to TP-Link.

If you are in communication with TP-Link support, send them that link.

Thanks @Alexandre.  - I already linked your thread in the very first contact with TP-link support- I though that just X20-DSL was not covered/listed and it will be a quick fix for known bug - but not :( appreciate your feedback