@RedBullet 

1. Do you run his Deco mesh in Access Point mode or in Router mode?

2. If Deco mesh runs in Router mode, does ISP router run in bridge mode or as a router?

3. Where is NAS connected to: ISP router, Main Deco, Satellite Deco?

4. How did you configure static IP address for NAS: on NAS, as IP reservation in Deco, as DHCP reservation in ISP router?

@Alexandre. Good questions. Should have included more info.

So the deco is in mesh mode. He is using a cable modem(not router). And the NAS is plugged into the main deco. And the deco is assigning

the reservation. The NAS is getting that through DHCP

@RedBullet 

You are not making it easy.  The "easy" would be if you set static IP address in NAS and it conflicted with IP allocation by Deco. What you have appears something different.

Let's assume for a moment it is not Deco fault, i.e., Deco does know how to allocate IP addresses. That would be reasonable assumption considering TP-Link company background, and that this software code of IP addresses management exists for eternity, without need to change it.

With that assumption, there are two somewhat unusual but likely possibilities I can come with. One is that you have device with MAC address same as NAS, meaning cloned MAC address, running on your network. Another is that you have device which has static IP address configured in it and it somehow conflicts with same reserved Ip at NAS. 

From your description, that device comes onoine infrequently and often late at night. Security camera, perhaps, that gets turned on during out of business hours?

-----------------

if I were in your place, this is how I would troubleshoot and investigate.

1. Change password for Main and Guest (if Guest is enabled) networks in Deco and don't share it with anyone, including your father. Configure all mobile devices in his place with new password, make sure they work and make sure they don't get conflicted IP. Check them in Deco, it is possible that NAS reports IP conflicts with delay.

2. Delete all "unknown" devices from Deco (good call) and configure alert for when these come online. Obviously, they shouldn't and couldn't as there is new WiFi password unless they are wired, and if they do - that is worth checking very closely.

(Steps 1 and 2 will take care of devices connected wirelessly, if you still have IP address conflict reported after that, focus on devices wired to Decos and/or his home network).

3. Every day, few hours before time IP conflict alert usually happens, reboot Main Deco to flush its logs (they are deleted on reboot) and that will make Deco not overwrite logs overnight. If IP conflict had happened, check logs. They could be quite helpful, for example this is a snippet of what they show in mine Main Deco when it ran in Router mode:

Sun Sep 20 23:44:25 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:60-XX-XX-XX-XX-B1 device_ip:192.168.68.101 device_hostname:H***3
Sun Sep 20 23:44:25 2020 daemon.err udhcpd[7370]: Sending OFFER of 192.168.68.101
Sun Sep 20 23:44:25 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:60-XX-XX-XX-XX-B1 device_ip:192.168.68.101 device_hostname:H***3
Sun Sep 20 23:44:25 2020 daemon.err udhcpd[7370]: Sending ACK to 192.168.68.101
Sun Sep 20 23:44:27 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:60-XX-XX-XX-XX-B1 device_ip:192.168.68.101 device_hostname:H***3
Sun Sep 20 23:44:27 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:D0-XX-XX-XX-XX-XX device_ip:192.168.68.103 device_hostname:R******

Sun Sep 20 23:44:27 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:44-XX-XX-XX-XX-C8 device_ip:192.168.68.104 device_hostname:Main-Floor
Sun Sep 20 23:44:27 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:44-XX-XX-XX-XX-14 device_ip:192.168.68.100 device_hostname:UNKNOWN
Sun Sep 20 23:44:27 2020 daemon.info udhcpd[7370]: fscanf_num:3 device_mac:98-XX-XX-XX-XX-XX device_ip:192.168.68.102 device_hostname:L******4
Sun Sep 20 23:44:27 2020 daemon.err udhcpd[7370]: Sending ACK to 192.168.68.102

I replaced portion of MAC addresses with XX and device names with **. You'll have cleartext MAC address and device names in your Main Deco logs. I suggest you download them to local disk every time for further analysis, there is such option in Deco web interface, so that they won't get overwritten.

@Alexandre. - yea, it puzzled me as well. I only have the one network, no Guest network. I figured I covered your possibility #2 by changing the IP address the Deco assigned the NAS, and sure enough a couple of days later it reported an IP conflict on the new IP address.

My dad doesn't have many devices; just a couple of phones, streaming devices, tablets, and computers.

Can I get to the logs for the Decos? I do wonder why they haven't noticed the conflict...

@RedBullet 

To get to Main Deco logs, just access its IP address in a browser. It'll open Web interface for Deco. If Main Deco IP is 192.168.68.1, open http://192.168.68.1 in a browser.

If you find you want to check logs from Satellite Decos, they are reachable by IP addresses of Satellite Decos.

Also, have you checked support forums for that NAS? It might produce false alarm, for example. 

@Alexandre. Unfortunately I am remote from him, so not easy for me to jump on his network. Though I guess I could VPN in using the NAS!

Seems unlikely it is false positive from the NAS. It started happen immediately after the update to the latest Deco firmware (which I updated on April 2nd)