To reactivate customers we must access:

Clients -> Clients.

Here we will see the list of clients registered by default, but we can perform a search of the clients that have been unsubscribed or have been unsubscribed in order to reactivate them. 

The search can be filtered by pressing the blue arrow to extend the filtering options:

In the state, we will select "Unsubscribe" and we will get the list of such users among which is the one we want to reactivate:

To reactivate them we have two options. The first is to press  and it will reactivate.

The second option is to press the icon and at the bottom of the client tab we will see the option Reactivate: