I'm Alexandre, AcyMailing CEO.
To be honest I don't get the point of this kind of thread. I can understand you're feeling frustrated because you're trying to do something and you don't find a way to do it. But in my opinion there is no need to write public posts like this saying our support is bad. It has side effects on us and that makes no sense.
I've read both discussions you had with Igor and I don't see anything he did in a bad way.
You were asking for advices about how to configure DKIM and he sent some useful links. Yes, you said you already read some documentation before, but we're not sure about what you read so we prefer send the right content in case you read something else.
As regards the issue configuring DKIM depends on tons of different things. Using Sendinblue I guess there is no need to set it as Sendinblue might do everything for you.
If you still think you need to set a DKIM then you should add a DNS "txt" entry from your hosting provider dashboard. As the process depends on your hosting provider and no one has the same than the others then we can specify the process. Best solution is to take the 2 values displayed in your DKIM configuration (in the AcyMailing configuration) then give those values to your hosting provider support and you'll be done as they will probably do it for you.