Who is the owner of that foreign network just needs to add your email (= tplink id) to the Administrators in their Deco app.
Then you you open your Deco app and login using your email, you can change in the hamburger menu bwteen your network and that foreign network (in the menu changes from owner to adminsitaror by then) and manage it.

I have my own deco network but also manage a foreign deco like this, it works perfectly.