Thank you for your reply.
Yes, that was my first impression too: it is not installed properly. On the other hand, I've never seen that second installer you told about.
I've disinstalled and reinstalled all but nothing better occurred: the Project7 Extension Manager is going on telling me, the Mysqli Server Behavior is "successfully" installed in DW CC2018 - where, there is no tracks at all of WebAssist menu.
Any further idea? Someone else with this issue?