PowerApps formula to submit a record if device is online - can be run on a timer OnEnd function for example.

PowerApps_TimerSubmitIfOnline.txt

If(
    Connection.Connected && !IsEmpty(LocalRecord),
    ForAll(
        LocalRecord,
        Patch(
            'My SharePoint list',
            Defaults('My SharePoint list'),
            {
                Title: Concatenate(
                    "Leave Request - ",
                    User().Email,
                    " - ",
                    Text(
                        Now(),
                        "[$-en-GB]dd/mm/yyyy hh:mm"
                    )
                ),
                Requestor: currentUser,
                RequestorStaffNumber: txtRequestorStaffNumber.Text,
                LeaveRequestType: LeaveType
            }
        )
    );
    Clear(LocalRecord);
    Navigate(
        ConfirmationScreen,
        ScreenTransition.Fade
    )
)