There are good reasons to make use of the article for which Charles provided a link (
How to number headings and figures in Appendixes in Microsoft Word | ShaunaKelly.com).
On the other hand, if you prefer not to apply numbering to headings and just use SEQ fields, that is going to work too. You will have to create two sequences, one for the main chapters and a separate sequence for the appendix. If you want
multilevel numbering, SEQ fields will be a far more difficult approach, though.