Auto increment doesn't work on primary key


I have a model called ObjetArchi.
The primary key field looks like this:

id_objet_archi = models.AutoField(primary_key=True)

As for my previous messages, I’m working on a legacy database. The primary key field already existed, hence the custom name.
When I try to add a new object through the admin form, the auto increment doesn’t seem to work and the following error appears:

null value in column “id_objet_archi” of relation “objetarchi” violates not-null constraint

I first set id_objet_archi as an IntegerField but the auto increment didn’t work either, so I was looking for the appropriate type.

Here is

from django.contrib.gis import admin
from django.contrib.gis.geos import Point
from .models import Illustrations, Materiau, ObjetArchi, Pays, ProvinceAntique, Region, Ville, Monument, CiteAntique, SiteAntique, Ensemble
class IllustrationsAdmin(admin.ModelAdmin):
    search_fields = ('legende', 'lien')

class IllusInline(admin.TabularInline):
    model = Illustrations
    fields = ('images', 'legende', 'principale')  

class ObjetArchiAdmin(admin.OSMGeoAdmin):
    list_display = ('titre', 'date')
    search_fields = (
    autocomplete_fields = ('prov_pays', 'prov_region', 'prov_departement','prov_ville', 'cons_pays', 'cons_region', 'cons_departement', 'cons_ville', 'materiau', 'ensemble', 'provinceantique', 'siteantique', 'citeantique', 'monument')
    exclude = ('id_objet_archi','lien_pdf', 'reference', 'version', 'observations', 'ref_repro', 'issn_objetarchi', 'flag_soumission_comite', 'lat', 'lon', 'cons_lieu', 'flag_del_objetarchi')
    inlines = (
    pnt = Point(5.855572,47.019214, srid=4326)
    default_lon, default_lat = pnt.coords

class PaysAdmin(admin.ModelAdmin):
    search_fields = ['nom']

class RegionAdmin(admin.ModelAdmin):
    search_fields = ['nom']

class DepartementAdmin(admin.ModelAdmin):
    search_fields = ['nom']

class VilleAdmin(admin.ModelAdmin):
    search_fields = ['nom']

class PAntiqueAdmin(admin.ModelAdmin):
    search_fields = ['nom']   

class SAntique(admin.ModelAdmin):
    search_fields = ['nom']

class CAntique(admin.ModelAdmin):
    search_fields = ['nom']

class MateriauAdmin(admin.ModelAdmin):
    search_fields = ['nom']

class EnsembleAdmin(admin.ModelAdmin):
    search_fields = ['nom']

class MonumentAdmin(admin.ModelAdmin):
    search_fields = ['nom'], ObjetArchiAdmin), PAntiqueAdmin), CAntique), SAntique), PaysAdmin), RegionAdmin), DepartementAdmin), VilleAdmin), MonumentAdmin), MateriauAdmin), EnsembleAdmin), IllustrationsAdmin) 

Is there something obviously wrong that I missed?

Thanks for your help!