Hi @LarsK1 

Thanks for posting in our business forum.

How many devices do you have? The sync speed varies when you have either too many config on the router or too many devices in your network.

Have some patience and wait for it to finish its application.

  @Clive_A thanks for your fast reply.

The controller controls 1xSwitch, 2xAPs and this router. 

We waited for at least 1 hour. The router was initially connected after 5 minutes and then began again, and again, and again. Therefore the connection to the Internet keeps on disconnecting.

Hi @LarsK1 

Thanks for posting in our business forum.

Have you ruled out the WAN duplicated IP?

Just enable WAN 2 and don't connect anything, will it be synced quickly?

  @Clive_A 

just tried, unfortunately still the same.

Hi @LarsK1 

Thanks for posting in our business forum.

LarsK1 wrote

  @Clive_A 

just tried, unfortunately still the same.

Back up your current config now for later use.

Disable that port and enable it again, will it recover? Or reset the router. Don't reset your controller. 
Lastly, try this firmware if you will.

https://community.tp-link.com/en/business/forum/topic/649884

  @Clive_A

I will try. Will report back