After about 4 months running #nixos I had to fix the #printing problems. After many hours fiddling & trying various fixes, the only way I got my #Brother #LAN local #network #printer to work was to use the following in my #configurationdotnix #nixconfiguration
configuration.nix
file:

services.printing.enable = true;
services.printing.drivers = [pkgs.brlaser];

with additionally the lines in the image below:

Complete configuration.nix is listed here
https://pastebin.com/SCNv2c3h
see lines 78-158