@Chris-Cross 

It would help if you ask your question one by one or list them.

1. Now I needed to reset the Router. I again activated the controller Setting in the Router and afterwards I again put it under controller of the software controller.

Please tell us how you reset it? By poking the reset hole or by controller function forgetting?

2. But now, it shows in the Omada Software configuring and this does not change. 

Have you tried refresh the page while the status keeping "configuring"? And you can reset it again and adopt it again.

3. All devices connected to the Router are showing that there is a internet connection, but the Connection does not work. What happed? 

How do you know that there's an internet connection? Did your devices get IP addresses? If you know the address of  gateway, you can try pinging it to test the connection between gateway and your devices.

4. Why does it only work in standalone mode now? Resetting again and unplugging it did not fix it.

Please remember that upplug and plug again won't reset it, will only reboot it. And how did you know that it work on standalone mode? You have logged into the management page? If so, you can provide a screenshot to us, instead of confusingly describing here. Sometimes, a screenshot can show most of the issues.

And it only works in standalone mode may because you didn't adopt it successfully. Or it wasn't found by your controller.

5. Another problem is, when I configure OpenVPN in standalone mode, I have a connection when I am doing this while I am in my own network. But when I do this from the Internet it connects, but I can not acces the internet. But even stranger, when I now try to connect again from my own network, it also connects, but now it also refuses to connect to the internet, even though this worked, before I tried this from the internet?!

I can't barely understand your situation from your description. I sincerely hope you can provide some screenshots of your configurations and settings. And when you finish setting up any VPN, it's better to test the connectivity of both ends.

  @Dahliana 

1. Always by poking the reset button. I have to. I have no acces to the Omada controller, because Internet is not working anymore and thus the Server is not connected anymore. And I can apparently not acces the Router directly anymore (like I would I standalone mode). It always shows password wrong, though it's right.

2.Yes, I tried to refresh, since i have no internet connection, it does not work.

3.The PCs connected to it, just show, that they have internet connection, but I can not access any homepage. I can ping the Router and other devices in my own network though.

4.I know, that replugging it does not reset it. Just thought it crashed. I know that standalone works, because after resetting it, I can set an new username and Password, as well as log in to the router. Also Internet immediately works again after resetting the Router. The problems start to appear, when I try to pair it to the Omada controller, wich is also directly in my Network. Even this worked bevor i had to reset it.

5. I add an Map of my setup. I first would like to fix the first problem.

Sorry that wrote a bit confusing. I am often in a hurry and try to first give a quick overview and later add things when there are questions (might not be the best approach)

  @Chris-Cross 

When the network gets down,have you tried plugging the FitzBox cable directly to your PC and test the connectivity of your network?

There's no problem within your LAN if you can ping the devices within your LAN. 

Could you tell me whether you can adopt your ER8411 after resetting? If not, what's the error message the management page showed?

  @Dahliana 

The FritzBox itself has Internet connection. I now tried to adopt while plugging in the controller in the FRITZBOx instead of the ER8411, so it always has a connection to the internet. Adoption works now that way and it shows on the Omada page adopted and connected. But still Internet for the ER8411 only works in Standalone mode. Paired to the Omada internet does not work.

  @KimcheeGUN 

I directly adopted the device after resetting. So I changed no configuration. Did not even set a password or user.

  @Chris-Cross 

What‘s the status of WAN port light and LAN port light and the status light on ER8411 when the internet is broken?

Do you mean the ER8411 only has an internet connection under standalone mode? When switching to Controller mode the internet won't work?