I believe that your device uses the Qu- and Quz-series of firmware blobs, not the 9000- or 9260-series; reference: https://bugs.launchpad.net/hwe-next/+bug/1833065 at comment #4. I believe that the later linux-firmware version 1.183 contains the referenced 48.xx versions. Please install it:
Disabling WiFi power management solved this problem for me. In the file /etc/NetworkManager/conf.d/default-wifi-powersave-on.conf, I changed it to look like this:
wifi.powersave = 2
Restart NetworkManager (sudo systemctl restart NetworkManager) and it should be go to go.
As James Reid says:
this has been fixed with the latest 5.0 kernel.
Now we have Ubuntu 18.04.3 with 5.0.0-31 kernel. Just run:
sudo apt-get update && sudo apt-get dist-upgrade -y
to get the last kernel version.