It seems that it can be declared as followed:
https://search.nixos.org/options?channel=24.05&from=0&size=50&sort=relevance&type=packages&query=ipv4.routes.
Some posts that may help:
- Networking.interfaces.<name>.ipv4.routes not working? (declared in config files)
-
Second Default Gateway - #6 by mmport80 (run as
systemdservice)