Hi, this is not really a kernel question, but a question about Subiquity installer, autoinstall option. It’s best to ask about it at Please test autoinstalls for 20.04! - #32 by mwhudson
In general, Ubuntu updates are rolling, meaning your kernel just like all the other packages will be upgraded. 5.4.0-125 is a very old kernel build which is no longer supported. It is not recommended to install individual kernel ABI packages, and instead you should only install the meta that will keep your systems updated (as is done by default).
Why are you trying to install an obsolete and security vulnerable build of v5.4 kernel, when latest abi is 150?