TabIndex is important for controls which are siblings of the same parent. However, if your TextBox and ComboBox controls are each inside different parents then their parent controls must have a the proper TabIndex.
In the Windows Forms Designer you can see which controls are children of which panels by bringing up the Document Outline. Go to View -> Other Windows -> Document Outline.
If each TextBox or ComboBox is directly inside a parent then its TabIndex doesn't matter, it can be 0. It's the parent (and possibly the parent's parent's) TabIndex which needs to be in order.