Disable fragment client

Hello together,

we are using an openvpn server with udp and the fragment 1400 option.
Now we have an iPhone which should also connect to the vpn server. The fragment option does not work for the iOS device, when I disable fragment on both sides its working.

But I have 30 users with the fragment option. Is it possible just to disable for one client/user (for the iOS device)?
I tried to setup different fragment options in /var/ipfire/ovpn/ccd/‘iosuser’
fragment 0
fragment none
fragment off

But nothing worked for me.

Thanks and with best regards