Problems with sync-clients from private ip range

Im’ running Nextcloud 12.0.1 on a raspberry pi, my router ist a fritzbox. Everything is working fine, if the clients connect from outside my private ip range.
e.g. the smartphone sync (windows mobile 10) does not work, if i am connected via wlan. If I disable wlan and connect through mobile data, everything is working fine.
Also I have Problems with the windows client, from time to time it can’t connect at all, sometimes it works - i have no clue how i can solve that problems. Please help!