@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.