@Piedes 

What are these devices? Is it possible they have MAC address randomization enabled - that would render IP reservation useless? Modern smartphones and tablets often do, while devices such as printer and Smart TV usually don't.

  @Alexandre. Here a list:

- Tesla wall connector

- 2 RPi

- 2 Synology

- 1 manageable switch

- 2 IP cameras

I don't think that these devices have MAC address randomization enabled.

Regards

  @Piedes 

I would suggest the following. First, delete all IP reservation entries you've entered manually. You said they are not working anyway.

Then, try this with one of Synology NAS: set its IP reservation in Deco mesh by picking it from the list of online devices, instead of entering MAC address manually. See if that NAS will keep its reserved IP address.

Hint: you can edit IP address assigned to the device to your liking, after you've created entry for that device in IP reservation list.

If that works, follow that process for the rest of devices you want to have static IP addresses assigned to.

  @Alexandre. Remove all reservations. Now it's not possible to add new reservation from client list or manually -_-'

I just hav a quick message saying "Failed". Do you know if it's possible to have some logs somewhere. Deco apps is a black box...

  @Piedes 

This appears to be an issue between Deco app and your account in TP-Link Deco cloud. Perhaps, by adding IP reservations manually, which is new feature, you've triggered some bug on their side. This can't be widespread issue, you are the only one reporting it here.

I would suggest escalation to TP-Link Support directly, through email from their support form. I can't give you a link because it is country specific, just Google for it. When submitting error report, make it simple. Subject line such as "Unable to configure Address Reservation for currently online device." In description, mention that reservation list is empty and exact error message you are getting.

If you would like to, you could try one of the following while waiting for their reply:

1. Logout from Deco app. Uninstall Deco app from smartphone. Reboot smartphone. Install Deco app. Login to Deco app. Try adding IP reservation from the list of connected devices (don't do manual entry of MAC address!).

2. If your Main Deco connected to ISP Router/Gateway, configure Deco mesh to run in Access Point mode. After that, you can set IP reservations in ISP Router/Gateway. Switching a Deco to Access Point (AP) Mode

You can't do that with X80-5G using cell network, please ignore this suggestion.

3. Delete your Deco mesh network. See this document, Scenario 2: How to delete a network on the Deco app. Create new network with different SSID (that is important). Add and configure Deco units to that new network. Try adding IP reservation from the list of connected devices (don't do manual entry of MAC address!).