Solved
Contacting with support I looked more carefully and noticed that in fact the free IS subscription was "linked" with a second Google account on my phone (picture). But it was impossible to link the Plus subscription to my phone: every install linked my phone to the free subscription.
Finally, the only solution was to remove the second Google account from my phone, install Kaspersky, link to My Kaspersky account and it worked. The Android app on my phone is now linked to my Plus subscription. Of course I add the second Google account again.
This is I think a Google "problem" : there was (at least for support and I) a way to "unlink" this free subscription from my second Google account.
