[numbers]{natbib} citet: page number outside brackets

I am trying to make a document (article class) using natbib, plainnat.
The citation style I want for citet is Author[#]:#page (e.g.: Debord [2]:102).

I tried the solution given here:
Natbib citet style: move optional argument outside bracket, that is

makeatletter
patchcmd{NAT@citex}
  {ifNAT@swaelseif*#2*elseNAT@cmt#2fi
   ifrelaxNAT@daterelaxelseNAT@@closefifi}
  {ifNAT@swaelseif*#2*elseifNAT@parelseNAT@cmt#2fifi
   ifrelaxNAT@daterelaxelseNAT@@close%
   if*#2*elseifNAT@parNAT@cmt#2fifififi}{}{}
makeatother

which actually works with usepackage{natbib} (author-year mode), but it doesn’t with the option [numbers]{natbib}, and I can only get Author[#:#page].

How can I solve this?
Thanks