The word in question ("public declaration" - Greek homologia; Strong's 3671) occurs with the same inflection in the NT only at 1Ti 6:12-13. But the word also occurs at: 2Co 9:13; Heb 3:1, 4:14.
The NWT renders it thusly:
2Co 9:13; 1Ti 6:12-13; Heb 4:14; 10:23 = "public declaration"; Heb 3:1 = "acknowledge." You can use this link and just change the translation in the upper right to see how other translations render it in all these verses.
Based on the context of these 6 verses the "confession" appears to be to both insiders and outsiders of the community of believers. I don't see anything that would require it to be only in a door-to-door ministry. Nor anything that would rule it out either as one way to give a 'confession.' (Footnote #17 in the NET speaks of 1Ti 6:12 happening at Timothy's baptism or ordination as a minister. - See here.)