To display additional standard/custom fields on your form, you have to follow Step-1 to Step-4 on the About Update Client Details tab.
- Step-1: Configure Field Sets - In this step, add the fields you want to display in the form into the ContactDetails fieldset.


- Step-2: Configure Intermediate Object - In this step, you need to create the same fields in the Contact Update object which you added in the Contact fieldset of Step-1.

- Step-3: Mapping the fields of Contact and Contact Update object - In this step, you need to map the fields of the Contact fieldset with the fields you created in Step-2.

- Step-4: Configure the Guest User Site -
Note - If you have already performed this step-4, you just need to go and provide the read, edit permissions to the newly added fields on the Contact Update object and the ones you added in the fieldset.
P.S. Discover Update Client Details, the easy way to keep client information up to date in Salesforce. Find it on AppExchange - https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3A00000EcsATUAZ