Comment 16 for bug 1704799

But you said yourself that "kernel 4.15.17 with linux-firmware 1.173 is working." 4.15 is currently the only supported kernel version in 18.04. Over time support for kernels for subsequent releases will be added via the linux-hwe packages, and linux-firmware will be updated accordingly, but as of right now there are not any subsequent releases.

The builds from http://kernel.ubuntu.com/~kernel-ppa/mainline/ are only for testing purposes and are not supported. If you wish to run one of those kernels you may encounter problems like missing firmware, but we generally won't be adding firmware specifically to make these work.

However, as it happens we have added some new iwlwifi firmware to the package currently in bionic-proposed (https://launchpad.net/ubuntu/+source/linux-firmware/1.173.1) for other reasons, so this might coincidentally provide a version that will work with 4.16.