Hi Thanks for the question
Generally in this situation , using linked templates, you need to ensure the linked templates are available in storage and use a SAS URL. This is because ARM is public-multi-tenant and cant "see" into your private resources (including repos)
If you cant or dont want to stage deployment templates in storage there's another approach which involves a secure approach to proxying the files from private repo described here
https://techcommunity.microsoft.com/t5/azure-architecture-blog/creating-a-custom-and-secure-azure-portal-offering/ba-p/3038344