Do they really need advertisers? I'm not sure why all these companies think the whatsapp route isn't possible. People are happy to pay a small yearly subscription for apps if they're truly doing something useful. While I don't personally use telegram, I'd argue it falls into the category of "useful enough to pay for".
They have 550M MAU, using a freemium model and assuming a 1% payment conversion, that's 5.5M monthly paying users. With a price ranging $1 to $10, that can be $66M ~ $660M ARR, at least they can try.