AC87u - how to force to non DFS channel

I'm using the AC87U (FW: 380.3459) already for sometime now. But lately I noticed that some of my 5GHz clients cannot find the AC87U anymore. The reason is that the AC87U is switching to a DFS channel (although I selected channel 36 as the channel to use) though my clients do not support DFS channels.

How can I force the router to stick to a non-DFS channel?

Thanks in advance.