Visible 0bda:a811 Realtek WiFi adapter and (I believe installed) device driver not connected

I answer for future reference, having found the question myself without answer. There are a couple of problems:

1. I had problems correctly identifying the chipset. It seems that this 0bda:a811 is the realtek chipset rtl8812au; at least for me the rtl8812au driver worked.
2. This chipset does not work for Ubuntu 18.04-1 out-of-the-box. Even the rtl8812au-dkms driver that you can install with apt-get does not work. sudo apt-get install rtl8812au-dkms seems to install version 4.3.8 (you can check with dkms status).
3. There is luckily a new version of the driver which does work, but you need to install it manually. See this answer. I don't fully understand if it's the driver that does not work or just some problem with the module loader.

To summarize with commands that will fix your problem:

git clone https://github.com/abperiasamy/rtl8812AU_8821AU_linux.git
cd rtl8812AU_8821AU_linux
sudo make -f Makefile.dkms install

After that, giving dkms status you should see a line like:

rtl8812au, 4.3.14, 4.15.0-30-generic, x86_64: installed

Reboot and it should work.