I do frequent the web, youtube, bus stops, tubes, trains and utterly exposed to all forms of marketing.
So from my perspective, this does seem like marketing failure from the start - at least from my perspective.
Though the only real new Bank in the UK to do well, traction wise would be Metro Bank and that's only due to them having a solid high street offering and more so niche in that they are open near on all hours. Also associated app and online works well.
But sorry, never even heard of N26 and is it initialy appears to tweak my interest that it may be something N64 related, would of at least had me read/look into it beyond that. Not even that happened, until now and only reinforces the lack of marketing carried out as testified by the aspect that I've never heard of them until now.
That's a shame as marketing is usually an area in which startups/companies tend to overdo more than not do at all.