How can I link lists across subsites ?

Hello,

I have some subsites with several lists and I would like to link these lists together.

Exemple : 

1. Subsite 1 : "Projects" with a list of all of the projects and a column "N°PROJECT"
2. Subsite 2 : "Customers" with a list of all of the customers and a column "Name-of-Customers"

• I would like to add a column "Name-of-customers" in the List "Projects"
• I would like to add a column "N°Project" in the List "Customers"

I already find the function "Create Cross-Site Lookup Site Column in SharePoint" https://www.sharepointdiary.com/2015/12/how-to-create-cross-site-lookup-site-column-in-sharepoint.html 

but in my exemple, the subsites are on the same level and the lists too...

Please help me... I'm looking for a solution (or I will have to put all of the list on the top level.... and I don't want)