Add it in dietpiEnv.txt to extraargs=, so that it looks like that:
extraargs=net.ifnames=0 isolcpus=3
With armbianEnv.txt it would be the same.
Add it in dietpiEnv.txt to extraargs=, so that it looks like that:
extraargs=net.ifnames=0 isolcpus=3
With armbianEnv.txt it would be the same.