OK, the mistake I was making is I was using wrong URL.
I was using this one:
and I thought it should be OK, but it wasn't. So I opened the whole banner in another tab in my browser (as you can see in ss nr 2)
and the URL was different to the one from SS nr 1. I tried it in ServerSettings.ini and it worked.
I was using this one:

and I thought it should be OK, but it wasn't. So I opened the whole banner in another tab in my browser (as you can see in ss nr 2)

and the URL was different to the one from SS nr 1. I tried it in ServerSettings.ini and it worked.