default or selected kernelversion does not have config value CONFIG_DECNET_ROUTER.
Result is shown for kernelversion 6.5.1

DECnet: router support


Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>DECnet: router support
In linux kernel since version 2.6.12  
Add support for turning your DECnet Endnode into a level 1 or 2
router. This is an experimental, but functional option. If you
do say Y here, then make sure that you also say Y to "Kernel/User
network link driver", "Routing messages" and "Network packet
filtering". The first two are required to allow configuration via
rtnetlink (you will need Alexey Kuznetsov's iproute2 package
from <>). The "Network packet
filtering" option will be required for the forthcoming routing daemon
to work.

See Documentation/networking/decnet.txt for more information.