Office 365 sharepoint / Group of people with limited access only to sub pages

Hello,

I've a question related to share point.

I used to do "below" before office 365, but can't do it anymore

My purpose is to create an office 365 sharepoint.

From the home page there are different subpages.

Each of those subpages would have very specific group with specific access.

Illustration:

From the home page, I open parameters/Permissions/Advanced permission settings

Then I create all my groups (without giving any rights at this level)

Then I go to the subpage, I open parameters/Permissions/Advanced permission settings and try to grant access to the corresponding group

And there, I've 2 issues:

1. My group is unknown
2. The window refer to the name of the home page, not the sub page

Would someone know how to proceed to have group of people able to only access sub page (subsite) with specific access?

Thanks

Fabrice