No, 4.2.1 doesn’t include kernel 5.4.

4.2.1 is still running on 4.4.60.

If I recall some discussion that happened on the forum a while ago, the issue is that the kernel hasn’t been compiled for the processor in the ATX-1800. Whereas the processor in the Beryl-AX is more “mainstream” and has been compiled already. I don’t know that it’s possible at this point to upgrade to kernel 5.4 at all on the ATX-1800.