@larrezz 

have you tested this firmware?

https://community.tp-link.com/en/business/forum/topic/710656

if it doesn't work, in some cases some problems can be solved if you do a factory reset of the device and readopt, you won't lose any settings so it doesn't hurt to try.

  @MR.S 

Thanks for the suggestions didn't know there was a beta version available. Will give it a try and let you know the results. 

 @MR.S 

I updated the firmware this morning and after the installation, all fixed ip addresses were pushed to the devices. After 2 hours the first tapo p115 device started to switch to a different IP. Should I maybe extend/disable the DHCP RENEW/RELEASE TIME? It's current set on 120 minutes. 

  @larrezz 

it's weird, I have about 25 P115 here myself but they don't have fixed ip, I set them to fixed ip on 3 units to test. I don't have an ER706W on this network, but an ER8411, I have another network with an ER706W, there are many devices that have a fixed IP via dhcp and there has never been a problem there either. dhcp lease of 120 minutes should be perfectly fine, but you can try to set it to 8 hours to see if it helps. I use 120 minutes leas time for the most on omada routers. 

  @MR.S 

At first, thanks for testing it on your own network. We are a couple of hours further and more devices are changing IP. My findings:

- It doesn't matter if they are connected with an AP or directly with the VPN router

- Wired devices are not changing IP, only wireless devices

- It happens on all SSID's (with each their own vlan and DHCP range)

- It happens also to other devices then Tapo P115s

Is there maybe an issue with the interaction between the VPN router and the controller?
Any other thoughts on this? 

  @larrezz 

I don't know what it could be then. now over 120 minutes have passed and the three P115s have the same ip address on my ER8411. all the devices that I have on my ER706w are wired devices, but I connected a WiFi device now and set the lease time to 10 minutes, we'll see. the device is connected to the ER706W WiFi.. 

It is not a P115 device but a google device conneted to WiFi

  @larrezz 

now I have tested a bit more with WiFi, everything looks normal, I have tested on different vlan, 2.4 and 5 ghz the wireless device gets the same ip that I have reserved all the time, so why you have problems I don't know.
I set the leas time to 3 minutes in order not to have to wait so long.

  @MR.S 

Thanks again for performing theses tests MR.S!

Just wondering, is your logfile looking the same like below?

There is no other information then these notices. There is also no other device running on 192.168.2.8 at the moment it is switching from 2.5 to 2.8. 

  @larrezz 

yes it looks like something is wrong, my log gives the same ip after i give the device a fixed ip. it seems your problem is something TP-Link should look into. have you tried to factory reset the router and readopt it? it might be an attempt. You can also try and run a Force Provision on the router first. If that doesn't help, I would do a factory reset by pressing the reset button for 20 seconds and readopt the router.

  @MR.S 

Thanks for checking. Currently doing the following actions:
- Upgrading the controller to 5.15 (Beta version)

- If this doesn't work, factory reset the router.

I already tried to force the settings earlier today and it worked for a couple of hours...