with the introduction of secureboot in UbuntuCore 20 by default for x86 hardware, all kernel modules need to be signed.
i fear the only two options you have here is to either ask the kernel team to include can-ahc in the ubuntu kernel or to get a brand store and maintain your own kernel snap for the hardware…
is the source code for can-ahc anywhere public so a kernel team member could take a look at it ?
also, you should file a bug on launchpad to ask for it: