Iphone and Ipad can't reach Server my own server

Hello, i recently set up a Synology DS218+ in my home network (Fritzbox!7590) and set up an owncloud (port 80/443/5000/5001 opended).

Now my Macbook, Windows 10 Laptop and Windows 10 Desktop can access the synology-server website, my owncloud and owncloud through the sync client.

But the IphoneX and Ipad can't reach the server. Neither through safari nor by app.

In the logfiles (owncloud) i clearly see the macbook and windows-pc's connecting. But no such requests from the iphone and ipad. Is there something special about the iphone und ipad so that they can't connect to my server?

Thanks in advance <3