Is it possible that because you are enabling Network Manager your IP address is coming from there? If that is the case, I think you would need to set the static IP inside network manager.
I have never setup a wifi connection declaritively but I am guessing if you would prefer that instead you would need to also setup the wifi connection declaritively using networking.supplicant or something similar.
Hopefully someone with more experience can come along and confirm or deny this theory.
What @dalto said is correct. This is because you are enabling network manager, and also trying to set the IP directly using the ip command (this is how the nixos network config works under the hood).
With network manager enabled, you should imagine that nm owns all interfaces, so you need to configure them through nm