Really,
from microsoft docs about TabIndex property:
TabPage controls are contained within TabControl controls, however, and do not receive focus individually as part of the form's TAB order. This property is therefore meaningless for this class.
So use SelectedIndex :
Next button :
yourTabControl.SelectedIndex += (yourTabControl.SelectedIndex == 5 ? 0 : 1);
Back button :
yourTabControl.SelectedIndex -= (yourTabControl.SelectedIndex == 0 ? 0 : 1);